diff --git a/index.js b/index.js
index cb2fea1..8c60e2b 100644
--- a/index.js
+++ b/index.js
@@ -58,7 +58,7 @@ module.exports = function(schema, options) {
                 const parentDoc = isSubdocument ? this.ownerDocument() : this;
                 const isNew = typeof parentDoc.isNew === "boolean" ? parentDoc.isNew : !isQuery;
 
-                const conditions = indexOptions.partialFilterExpression || {};
+                let conditions = {};
                 each(paths, name => {
                   let pathValue;
 
@@ -113,6 +113,7 @@ module.exports = function(schema, options) {
                 if (model.baseModelName && indexOptions.partialFilterExpression === null) {
                   model = model.db.model(model.baseModelName);
                 }
+                conditions = {$and: [conditions, indexOptions.partialFilterExpression || {}]};
 
                 model.find(conditions).countDocuments((err, count) => {
                   resolve(count === 0);
diff --git a/package.json b/package.json
index 6ac59e6..e1ea15f 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "mongoose-unique-validator",
-  "version": "2.1.0",
+  "version": "2.1.1",
   "description": "mongoose-unique-validator is a plugin which adds pre-save validation for unique fields within a Mongoose schema.",
   "main": "index.js",
   "scripts": {