diff --git a/.changeset/hot-foxes-perform.md b/.changeset/hot-foxes-perform.md
new file mode 100644
index 0000000..f8534ee
--- /dev/null
+++ b/.changeset/hot-foxes-perform.md
@@ -0,0 +1,5 @@
+---
+"@remix-run/web-fetch": patch
+---
+
+fix: Allow subclass methods of Headers to be called correctly
diff --git a/packages/fetch/src/headers.js b/packages/fetch/src/headers.js
index 86ae2aa..4095c4a 100644
--- a/packages/fetch/src/headers.js
+++ b/packages/fetch/src/headers.js
@@ -104,55 +104,6 @@ export default class Headers extends URLSearchParams {
 				[];
 
 		super(result);
-
-		// Returning a Proxy that will lowercase key names, validate parameters and sort keys
-		// eslint-disable-next-line no-constructor-return
-		return new Proxy(this, {
-			get(target, p, receiver) {
-				switch (p) {
-					case 'append':
-					case 'set':
-						/**
-						 * @param {string} name
-						 * @param {string} value
-						 */
-						return (name, value) => {
-							validateHeaderName(name);
-							validateHeaderValue(name, String(value));
-							return URLSearchParams.prototype[p].call(
-								target,
-								String(name).toLowerCase(),
-								String(value)
-							);
-						};
-
-					case 'delete':
-					case 'has':
-					case 'getAll':
-						/**
-						 * @param {string} name
-						 */
-						return name => {
-							validateHeaderName(name);
-							// @ts-ignore
-							return URLSearchParams.prototype[p].call(
-								target,
-								String(name).toLowerCase()
-							);
-						};
-
-					case 'keys':
-						return () => {
-							target.sort();
-							return new Set(URLSearchParams.prototype.keys.call(target)).keys();
-						};
-
-					default:
-						return Reflect.get(target, p, receiver);
-				}
-			}
-			/* c8 ignore next */
-		});
 	}
 
 	get [Symbol.toStringTag]() {
@@ -181,6 +132,56 @@ export default class Headers extends URLSearchParams {
 		return value;
 	}
 
+	/**
+	 * @param {string} name
+	 */
+	getAll(name) {
+		validateHeaderName(name);
+		return super.getAll(String(name).toLowerCase());
+	}
+
+	/**
+	 * @param {string} name
+	 * @param {string} value
+	 */
+	append(name, value) {
+		validateHeaderName(name);
+		validateHeaderValue(name, String(value));
+		return super.append(
+			String(name).toLowerCase(),
+			String(value)
+		);
+	}
+
+	/**
+	 * @param {string} name
+	 */
+	delete(name) {
+		validateHeaderName(name);
+		return super.delete(String(name).toLowerCase());
+	}
+
+	/**
+	 * @param {string} name
+	 */
+	has(name) {
+		validateHeaderName(name);
+		return super.has(String(name).toLowerCase());
+	}
+
+	/**
+	 * @param {string} name
+	 * @param {string} value
+	 */
+	set(name, value) {
+		validateHeaderName(name);
+		validateHeaderValue(name, String(value));
+		return super.set(
+			String(name).toLowerCase(),
+			String(value)
+		);
+	}
+
 	/**
 	 * @param {(value: string, key: string, parent: this) => void} callback
 	 * @param {any} thisArg
@@ -199,6 +200,11 @@ export default class Headers extends URLSearchParams {
 		}
 	}
 
+	keys() {
+		this.sort();
+		return new Set(super.keys()).keys();
+	}
+
 	/**
 	 * @returns {IterableIterator<string>}
 	 */
@@ -272,7 +278,18 @@ export default class Headers extends URLSearchParams {
  */
 Object.defineProperties(
 	Headers.prototype,
-	['get', 'entries', 'forEach', 'values'].reduce((result, property) => {
+	[
+		'append',
+		'delete',
+		'entries',
+		'forEach',
+		'get',
+		'getAll',
+		'has',
+		'keys',
+		'set',
+		'values'
+	].reduce((result, property) => {
 		result[property] = {enumerable: true};
 		return result;
 	}, /** @type {Record<string, {enumerable:true}>} */ ({}))
diff --git a/packages/fetch/test/headers.js b/packages/fetch/test/headers.js
index 8f9e168..11e44f0 100644
--- a/packages/fetch/test/headers.js
+++ b/packages/fetch/test/headers.js
@@ -347,4 +347,34 @@ describe('Headers', () => {
 		// eslint-disable-next-line quotes
 		expect(util.format(headers)).to.equal("{ a: [ '1', '3' ], b: '2', host: 'thehost' }");
 	});
+
+	it('should have the correct prototype chain', () => {
+		const headers = new Headers();
+
+		expect(headers).to.be.instanceOf(Headers);
+		expect(Object.getPrototypeOf(headers)).to.equal(Headers.prototype);
+	});
+
+	it('should have the correct prototype chain when extended', () => {
+		class MyHeaders extends Headers {}
+
+		const headers = new MyHeaders();
+
+		expect(headers).to.be.instanceOf(MyHeaders);
+		expect(headers).to.be.instanceOf(Headers);
+		expect(Object.getPrototypeOf(headers)).to.equal(MyHeaders.prototype);
+	});
+
+	it('should call the method of the subclass', () => {
+		class MyHeaders extends Headers {
+			append(_name, _value) {
+				return 'subclass method called';
+			}
+		}
+
+		const headers = new MyHeaders();
+		const result = headers.append('Content-Type', 'application/json');
+
+		expect(result).to.equal('subclass method called');
+	});
 });