diff --git a/example/package.json b/example/package.json
index 34b696b..e6c2851 100644
--- a/example/package.json
+++ b/example/package.json
@@ -4,7 +4,8 @@
   "main": "index.js",
   "license": "MIT",
   "scripts": {
-    "start": "webpack-dev-server"
+    "start": "webpack-dev-server",
+    "build": "webpack --mode development"
   },
   "dependencies": {
     "react": "^16.13.1",
diff --git a/example/src/App.js b/example/src/App.js
index faf1065..5b39b1b 100644
--- a/example/src/App.js
+++ b/example/src/App.js
@@ -3,13 +3,23 @@ import stylemug from 'stylemug';
 import { globalStyles } from './globals';
 
 const styles = stylemug.create({
+  foo: 'bar',
   title: {
     fontSize: '31px',
     fontFamily: 'courier',
-    color: '#444',
+    color: 'green',
   },
   titleRed: {
     color: 'red',
+    '&:hoverr': {
+      color: 'red',
+    },
+  },
+});
+
+const secondaryStyles = stylemug.create({
+  color: {
+    color: '#444',
   },
 });
 
@@ -22,7 +32,11 @@ export default function App() {
 
   return (
     <div className={styles(globalStyles.container)}>
-      <h1 className={styles('title', show && 'titleRed')}>Hello World</h1>
+      <h1
+        className={styles('title', secondaryStyles.color, show && 'titleRed')}
+      >
+        Hello World
+      </h1>
       <button onClick={onClick}>Click</button>
     </div>
   );
diff --git a/packages/babel-stylemug-plugin/src/__test__/__snapshots__/babel.test.js.snap b/packages/babel-stylemug-plugin/src/__test__/__snapshots__/babel.test.js.snap
index 25c692f..b4e37d3 100644
--- a/packages/babel-stylemug-plugin/src/__test__/__snapshots__/babel.test.js.snap
+++ b/packages/babel-stylemug-plugin/src/__test__/__snapshots__/babel.test.js.snap
@@ -1,5 +1,18 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`babel plugin - reports should replace create argument 1`] = `
+"\\"use strict\\";
+
+var _stylemug = _interopRequireDefault(require(\\"stylemug\\"));
+
+function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { \\"default\\": obj }; }
+
+var styles = _stylemug[\\"default\\"].create({}, {
+  sourceLinesRange: \\"In lines 4 to 4\\",
+  messages: [\\"mockError1\\", \\"mockError2\\"]
+});"
+`;
+
 exports[`babel plugin should error when failed to resolve 1`] = `
 "\\"use strict\\";
 
@@ -13,7 +26,10 @@ function _defineProperty(obj, key, value) { if (key in obj) { Object.definePrope
 
 var styles = _stylemug[\\"default\\"].create(_defineProperty({}, _mock.selector, {
   color: 'red'
-}), \\"Failed to evaluate the following stylesheet: \\\\n\\\\nstylemug.create({\\\\n  [selector]: {\\\\n    color: 'red'\\\\n  }\\\\n})\\\\n\\\\nMake sure your stylesheet is statically defined.\\");"
+}), {
+  sourceLinesRange: \\"In lines 5 to 9\\",
+  messages: [\\"Failed to evaluate the following stylesheet: \\\\n\\\\nstylemug.create({\\\\n  [selector]: {\\\\n    color: 'red'\\\\n  }\\\\n})\\\\n\\\\nMake sure your stylesheet is statically defined.\\"]
+});"
 `;
 
 exports[`babel plugin should replace create argument 1`] = `
diff --git a/packages/babel-stylemug-plugin/src/__test__/babel.test.js b/packages/babel-stylemug-plugin/src/__test__/babel.test.js
index 320457a..cfe6b16 100644
--- a/packages/babel-stylemug-plugin/src/__test__/babel.test.js
+++ b/packages/babel-stylemug-plugin/src/__test__/babel.test.js
@@ -1,17 +1,25 @@
 import { transform } from '@babel/core';
+import { compileSchema } from 'stylemug-compiler';
 import { babelPlugin as plugin } from '../babel';
 
 jest.mock('stylemug-compiler', () => ({
-  compileSchema: () => ({
-    className: {
-      hash: {
-        keyId: 'id',
-      },
-    },
-  }),
+  compileSchema: jest.fn(),
 }));
 
 describe('babel plugin', () => {
+  beforeEach(() => {
+    compileSchema.mockImplementationOnce(() => ({
+      result: {
+        className: {
+          hash: {
+            keyId: 'id',
+          },
+        },
+      },
+      reports: [],
+    }));
+  });
+
   it('should replace create argument', () => {
     const example = `
       import stylemug from 'stylemug';
@@ -73,3 +81,24 @@ describe('babel plugin', () => {
     expect(code).toMatchSnapshot();
   });
 });
+
+describe('babel plugin - reports', () => {
+  it('should replace create argument', () => {
+    compileSchema.mockImplementationOnce(() => ({
+      result: {},
+      reports: [{ message: 'mockError1' }, { message: 'mockError2' }],
+    }));
+
+    const example = `
+      import stylemug from 'stylemug';
+
+      const styles = stylemug.create({});
+    `;
+
+    const { code } = transform(example, {
+      plugins: [plugin],
+    });
+
+    expect(code).toMatchSnapshot();
+  });
+});
diff --git a/packages/babel-stylemug-plugin/src/babel.js b/packages/babel-stylemug-plugin/src/babel.js
index 46722c6..a284ff9 100644
--- a/packages/babel-stylemug-plugin/src/babel.js
+++ b/packages/babel-stylemug-plugin/src/babel.js
@@ -5,9 +5,39 @@ import { compileSchema } from 'stylemug-compiler';
 export function babelPlugin(babel) {
   const t = babel.types;
 
-  function defineError(path, msg) {
+  function wrapReports(path, reports) {
+    if (!Array.isArray(reports)) {
+      reports = [reports];
+    }
+    if (!reports.length) {
+      return;
+    }
+
     const node = t.cloneDeep(path.node);
-    node.arguments[1] = t.stringLiteral(msg);
+    const objectChilds = [
+      t.objectProperty(
+        t.identifier('sourceLinesRange'),
+        t.stringLiteral(
+          'In lines ' + node.loc.start.line + ' to ' + node.loc.end.line
+        )
+      ),
+      t.objectProperty(
+        t.identifier('messages'),
+        t.arrayExpression(
+          reports.map((report) => t.stringLiteral(report.message))
+        )
+      ),
+    ];
+    if (path.hub.file.opts.filename) {
+      objectChilds.push(
+        t.objectProperty(
+          t.identifier('fileName'),
+          t.stringLiteral(path.hub.file.opts.filename || 'unknown')
+        )
+      );
+    }
+
+    node.arguments[1] = t.objectExpression(objectChilds);
     path.replaceWith(node);
   }
 
@@ -34,22 +64,24 @@ export function babelPlugin(babel) {
         references.forEach((reference) => {
           const local = reference.parentPath.parentPath;
 
-          let sheet = evaluateSimple(local.get('arguments')[0]);
+          const sheet = evaluateSimple(local.get('arguments')[0]);
           if (!sheet.confident) {
-            defineError(
-              local,
-              'Failed to evaluate the following stylesheet: \n\n' +
+            wrapReports(local, {
+              message:
+                'Failed to evaluate the following stylesheet: \n\n' +
                 local.toString() +
                 '\n\n' +
-                'Make sure your stylesheet is statically defined.'
-            );
+                'Make sure your stylesheet is statically defined.',
+            });
             return;
           }
-          sheet = compileSchema(sheet.value);
+
+          const { result, reports } = compileSchema(sheet.value);
+          wrapReports(local, reports);
 
           const nextLocal = t.cloneDeep(local.node);
           nextLocal.arguments[0] = t.objectExpression(
-            Object.entries(sheet).map(([name, rules]) => {
+            Object.entries(result).map(([name, rules]) => {
               return t.objectProperty(
                 t.identifier(name),
                 t.objectExpression(
diff --git a/packages/stylemug-compiler/src/__test__/__snapshots__/compile-context.test.js.snap b/packages/stylemug-compiler/src/__test__/__snapshots__/compile-context.test.js.snap
new file mode 100644
index 0000000..ee132ae
--- /dev/null
+++ b/packages/stylemug-compiler/src/__test__/__snapshots__/compile-context.test.js.snap
@@ -0,0 +1,63 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`compile should assert an invalid rule value 1`] = `
+Array [
+  Object {
+    "message": "Invalid type for color",
+  },
+]
+`;
+
+exports[`compile should assert an invalid sheet 1`] = `
+Object {
+  "default": Object {
+    "b765dfa68": Object {
+      "children": "",
+      "key": "backgroundColor",
+      "keyId": "bb7c5a6d0",
+      "media": undefined,
+      "value": "yellow",
+    },
+    "cb34cd11f": Object {
+      "children": "",
+      "key": "color",
+      "keyId": "c3d7e6258",
+      "media": undefined,
+      "value": "red",
+    },
+  },
+}
+`;
+
+exports[`compile should assert an invalid sheet 2`] = `
+Array [
+  Object {
+    "message": "Classname with key foo is not an object",
+  },
+]
+`;
+
+exports[`compile should assert invalid pseudo classes 1`] = `
+Array [
+  Object {
+    "message": "&:iAmAnInvalidPseudoClass is an invalid CSS pseudo class / element",
+  },
+  Object {
+    "message": "Invalid type for color",
+  },
+]
+`;
+
+exports[`compile should report multiple errors 1`] = `
+Array [
+  Object {
+    "message": "Classname with key foo is not an object",
+  },
+  Object {
+    "message": "&:iAmAnInvalidPseudoClass is an invalid CSS pseudo class / element",
+  },
+  Object {
+    "message": "Invalid type for color",
+  },
+]
+`;
diff --git a/packages/stylemug-compiler/src/__test__/compile-context.test.js b/packages/stylemug-compiler/src/__test__/compile-context.test.js
new file mode 100644
index 0000000..ceddd11
--- /dev/null
+++ b/packages/stylemug-compiler/src/__test__/compile-context.test.js
@@ -0,0 +1,52 @@
+import { compileSchema } from '../compile';
+
+describe('compile', () => {
+  it('should assert an invalid sheet', () => {
+    const { result, reports } = compileSchema({
+      default: {
+        color: 'red',
+        backgroundColor: 'yellow',
+      },
+      // The rule below fails.
+      foo: 'bar',
+    });
+
+    expect(result).toMatchSnapshot();
+    expect(reports).toMatchSnapshot();
+  });
+
+  it('should assert an invalid rule value', () => {
+    const { reports } = compileSchema({
+      default: {
+        color: true,
+      },
+    });
+
+    expect(reports).toMatchSnapshot();
+  });
+
+  it('should assert invalid pseudo classes', () => {
+    const { reports } = compileSchema({
+      default: {
+        '&:iAmAnInvalidPseudoClass': {
+          color: true,
+        },
+      },
+    });
+
+    expect(reports).toMatchSnapshot();
+  });
+
+  it('should report multiple errors', () => {
+    const { reports } = compileSchema({
+      foo: 'bar',
+      default: {
+        '&:iAmAnInvalidPseudoClass': {
+          color: true,
+        },
+      },
+    });
+
+    expect(reports).toMatchSnapshot();
+  });
+});
diff --git a/packages/stylemug-compiler/src/__test__/compile.test.js b/packages/stylemug-compiler/src/__test__/compile.test.js
index 4c5c775..e001048 100644
--- a/packages/stylemug-compiler/src/__test__/compile.test.js
+++ b/packages/stylemug-compiler/src/__test__/compile.test.js
@@ -2,7 +2,7 @@ import { compileSchema, compileSelectors } from '../compile';
 
 describe('compile', () => {
   it('should compile schema', () => {
-    const result = compileSchema({
+    const { result } = compileSchema({
       default: {
         color: 'red',
         backgroundColor: 'yellow',
@@ -18,16 +18,8 @@ describe('compile', () => {
     expect(result).toMatchSnapshot();
   });
 
-  it('should compile selectors with number as value', () => {
-    const result = compileSelectors({
-      fontSize: 12,
-    });
-
-    expect(result).toMatchSnapshot();
-  });
-
   it('should compile nested selectors', () => {
-    const result = compileSchema({
+    const { result } = compileSchema({
       default: {
         color: 'red',
 
@@ -41,7 +33,7 @@ describe('compile', () => {
   });
 
   it('should compile nested media query', () => {
-    const result = compileSchema({
+    const { result } = compileSchema({
       default: {
         color: 'red',
 
@@ -53,4 +45,11 @@ describe('compile', () => {
 
     expect(result).toMatchSnapshot();
   });
+
+  it('should compile selectors with number as value', () => {
+    const result = compileSelectors({
+      fontSize: 12,
+    });
+    expect(result).toMatchSnapshot();
+  });
 });
diff --git a/packages/stylemug-compiler/src/compile.js b/packages/stylemug-compiler/src/compile.js
index 869f88c..542cf4b 100644
--- a/packages/stylemug-compiler/src/compile.js
+++ b/packages/stylemug-compiler/src/compile.js
@@ -1,6 +1,8 @@
 import fnv1a from 'fnv1a';
 import { save as extractorSave } from './extractor';
 import { extractShorthands } from './shorthands';
+import { createContext } from './context';
+import { PseudoClasses, PseudoElements } from './static';
 
 function hash(str) {
   let hash = fnv1a(str).toString(16);
@@ -9,54 +11,70 @@ function hash(str) {
   return `${prepend}${hash}`;
 }
 
-export function compileSelectors(selectors, children, media) {
+export function compileSelectors(selectors, children, media, context) {
   let result = {};
 
   selectors = extractShorthands(selectors);
 
   for (let key in selectors) {
     const value = selectors[key];
+    const type = typeof value;
 
-    switch (typeof value) {
-      case 'object':
-        // @media
-        if (/^@/.test(key)) {
-          result = Object.assign(
-            result,
-            compileSelectors(value, children, key)
-          );
-        }
-        // &:focus
-        else {
-          const child = key.replace(/&/g, '');
-          result = Object.assign(
-            result,
-            compileSelectors(value, children + child, media)
-          );
-        }
+    // @media
+    if (/^@/.test(key)) {
+      if (type !== 'object') {
+        context.report('Rule with key ' + key + ' is not an object');
         continue;
+      }
+      result = Object.assign(
+        result,
+        compileSelectors(value, children, key, context)
+      );
+      continue;
+    }
+
+    // &:focus
+    // &::after
+    if (/^&/.test(key)) {
+      const child = key.replace(/&/g, '');
 
-      case 'number':
-      case 'string':
-        const mediaKey = media || '';
-        const className = hash(key + value + children + mediaKey);
-
-        const entry = {
-          // Provide a key id to match selectors setting the same key,
-          // for example, when for a given key, two classes apply with both
-          // backgroundColor as propery, we can filter out the first one by specificity.
-          keyId: hash(key + children + mediaKey),
-          key,
-          value,
-          children,
-          media,
-        };
-
-        extractorSave(className, entry);
-
-        result[className] = entry;
+      if (!PseudoClasses.includes(child) && !PseudoElements.includes(child)) {
+        context.report(key + ' is an invalid CSS pseudo class / element');
+      }
+      if (type !== 'object') {
+        context.report('Rule with key ' + key + ' is not an object');
         continue;
+      }
+
+      result = Object.assign(
+        result,
+        compileSelectors(value, children + child, media, context)
+      );
+      continue;
+    }
+
+    if (type === 'number' || type === 'string') {
+      const mediaKey = media || '';
+      const className = hash(key + value + children + mediaKey);
+
+      const entry = {
+        // Provide a key id to match selectors setting the same key,
+        // for example, when for a given key, two classes apply with both
+        // backgroundColor as propery, we can filter out the first one by specificity.
+        keyId: hash(key + children + mediaKey),
+        key,
+        value,
+        children,
+        media,
+      };
+
+      extractorSave(className, entry);
+
+      result[className] = entry;
+      continue;
     }
+
+    context.report('Invalid type for ' + key);
   }
 
   return result;
@@ -64,10 +82,16 @@ export function compileSelectors(selectors, children, media) {
 
 export function compileSchema(schema) {
   const result = {};
+  const context = createContext();
 
   for (let key in schema) {
-    result[key] = compileSelectors(schema[key], '');
+    if (typeof schema[key] !== 'object') {
+      context.report('Classname with key ' + key + ' is not an object');
+      continue;
+    }
+
+    result[key] = compileSelectors(schema[key], '', undefined, context);
   }
 
-  return result;
+  return { result, reports: context.getReports() };
 }
diff --git a/packages/stylemug-compiler/src/context.js b/packages/stylemug-compiler/src/context.js
new file mode 100644
index 0000000..c813de5
--- /dev/null
+++ b/packages/stylemug-compiler/src/context.js
@@ -0,0 +1,18 @@
+export function createContext() {
+  const reports = [];
+
+  function report(message) {
+    reports.push({
+      message,
+    });
+  }
+
+  function getReports() {
+    return reports;
+  }
+
+  return {
+    report,
+    getReports,
+  };
+}
diff --git a/packages/stylemug-compiler/src/static.js b/packages/stylemug-compiler/src/static.js
new file mode 100644
index 0000000..93c8d6d
--- /dev/null
+++ b/packages/stylemug-compiler/src/static.js
@@ -0,0 +1,41 @@
+export const PseudoClasses = [
+  ':active',
+  ':checked',
+  ':disabled',
+  ':empty',
+  ':enabled',
+  ':first-child',
+  ':first-of-type',
+  ':focus',
+  ':hover',
+  ':in-range',
+  ':invalid',
+  ':lang(language)',
+  ':last-child',
+  ':last-of-type',
+  ':link',
+  ':not(selector)',
+  ':nth-child(n)',
+  ':nth-last-child(n)',
+  ':nth-last-of-type(n)',
+  ':nth-of-type(n)',
+  ':only-of-type',
+  ':only-child',
+  ':optional',
+  ':out-of-range',
+  ':read-only',
+  ':read-write',
+  ':required',
+  ':root',
+  ':target',
+  ':valid',
+  ':visited',
+];
+
+export const PseudoElements = [
+  '::after',
+  '::before',
+  '::first-letter',
+  '::first-line',
+  '::selection',
+];
diff --git a/packages/stylemug/src/__test__/runtime.test.js b/packages/stylemug/src/__test__/runtime.test.js
index a101e40..b3fd21b 100644
--- a/packages/stylemug/src/__test__/runtime.test.js
+++ b/packages/stylemug/src/__test__/runtime.test.js
@@ -43,21 +43,6 @@ describe('runtime', () => {
     });
   });
 
-  describe('resolver warnings', () => {
-    it('should warn when a lookup failed', () => {
-      const styles = runtime.create({
-        foo: {
-          color: 'red',
-        },
-      });
-      styles('unknown');
-
-      expect(warn).toHaveBeenCalledWith(
-        'The class name "unknown" does not exist in your stylesheet. Check your stylemug.create({}) definition.'
-      );
-    });
-  });
-
   describe('compiler warnings', () => {
     it('should warn if a compiler error is thrown', () => {
       runtime.create(
diff --git a/packages/stylemug/src/__test__/warn.test.js b/packages/stylemug/src/__test__/warn.test.js
deleted file mode 100644
index 3a6fb74..0000000
--- a/packages/stylemug/src/__test__/warn.test.js
+++ /dev/null
@@ -1,10 +0,0 @@
-import warn from '../warn';
-
-global.console.warn = jest.fn();
-
-it('should log a warning message', () => {
-  warn('Something bad happened');
-  expect(global.console.warn).toHaveBeenCalledWith(
-    '[stylemug] Something bad happened'
-  );
-});
diff --git a/packages/stylemug/src/index.js b/packages/stylemug/src/index.js
index 935ec74..daa7c9d 100644
--- a/packages/stylemug/src/index.js
+++ b/packages/stylemug/src/index.js
@@ -1,14 +1,9 @@
 import warn from './warn';
 
-const noop = () => {};
-
 export default {
   create(schema, error) {
-    if (error) {
-      if (__DEV__) {
-        warn(error);
-      }
-      return noop;
+    if (__DEV__ && error) {
+      warn(error);
     }
 
     const resolver = (...classNames) => {
@@ -24,14 +19,6 @@ export default {
           maps.push(className);
         }
         if (typeof className === 'string') {
-          if (__DEV__ && !schema[className]) {
-            warn(
-              'The class name "' +
-                className +
-                '" does not exist in your stylesheet. Check your ' +
-                'stylemug.create({}) definition.'
-            );
-          }
           maps.push(schema[className]);
         }
       }
diff --git a/packages/stylemug/src/warn.js b/packages/stylemug/src/warn.js
index ec6e9eb..69ce069 100644
--- a/packages/stylemug/src/warn.js
+++ b/packages/stylemug/src/warn.js
@@ -1,9 +1,15 @@
-const warnings = {};
+export default function warn(error) {
+  let message = 'There are warnings in your stylesheet:\n\n';
 
-export default function warn(str) {
-  if (warnings[warn]) {
-    return;
+  if (error.fileName) {
+    message += error.fileName + '\n';
+    message += '  ' + error.sourceLinesRange;
+    message += '\n\n';
   }
-  warnings[warn] = true;
-  console.warn('[stylemug] ' + str);
+
+  error.messages.forEach((msg, i) => {
+    message += i + 1 + ') ' + msg + '\n';
+  });
+
+  console.warn('[stylemug]', message);
 }
diff --git a/scripts/dev b/scripts/dev
index 120c50a..6b36ed0 100755
--- a/scripts/dev
+++ b/scripts/dev
@@ -7,7 +7,7 @@ spawn('scripts/build', {
   stdio: 'inherit',
 });
 
-spawn('yarn', ['start'], {
+spawn('yarn', ['build'], {
   stdio: 'inherit',
   cwd: path.resolve('./example'),
 });