diff --git a/bench/parser.benchmark.js b/bench/parser.benchmark.js
index ce295ccd9..dd97701af 100644
--- a/bench/parser.benchmark.js
+++ b/bench/parser.benchmark.js
@@ -29,14 +29,14 @@ const pingFrame1 = Buffer.concat(
 );
 
 const textFrame = Buffer.from('819461616161' + '61'.repeat(20), 'hex');
-const pingFrame2 = Buffer.from('8900', 'hex');
+const pingFrame2 = Buffer.from('8980146e915a', 'hex');
 const binaryFrame1 = createBinaryFrame(125);
 const binaryFrame2 = createBinaryFrame(65535);
 const binaryFrame3 = createBinaryFrame(200 * 1024);
 const binaryFrame4 = createBinaryFrame(1024 * 1024);
 
 const suite = new benchmark.Suite();
-const receiver = new Receiver();
+const receiver = new Receiver('nodebuffer', {}, true);
 
 suite.add('ping frame (5 bytes payload)', {
   defer: true,
diff --git a/lib/receiver.js b/lib/receiver.js
index 3a8b92a6b..57daa725d 100644
--- a/lib/receiver.js
+++ b/lib/receiver.js
@@ -30,14 +30,17 @@ class Receiver extends Writable {
    *
    * @param {String} binaryType The type for binary data
    * @param {Object} extensions An object containing the negotiated extensions
+   * @param {Boolean} isServer Specifies whether to operate in client or server
+   *     mode
    * @param {Number} maxPayload The maximum allowed message length
    */
-  constructor(binaryType, extensions, maxPayload) {
+  constructor(binaryType, extensions, isServer, maxPayload) {
     super();
 
     this._binaryType = binaryType || BINARY_TYPES[0];
     this[kWebSocket] = undefined;
     this._extensions = extensions || {};
+    this._isServer = !!isServer;
     this._maxPayload = maxPayload | 0;
 
     this._bufferedBytes = 0;
@@ -225,6 +228,16 @@ class Receiver extends Writable {
     if (!this._fin && !this._fragmented) this._fragmented = this._opcode;
     this._masked = (buf[1] & 0x80) === 0x80;
 
+    if (this._isServer) {
+      if (!this._masked) {
+        this._loop = false;
+        return error(RangeError, 'MASK must be set', true, 1002);
+      }
+    } else if (this._masked) {
+      this._loop = false;
+      return error(RangeError, 'MASK must be clear', true, 1002);
+    }
+
     if (this._payloadLength === 126) this._state = GET_PAYLOAD_LENGTH_16;
     else if (this._payloadLength === 127) this._state = GET_PAYLOAD_LENGTH_64;
     else return this.haveLength();
diff --git a/lib/websocket.js b/lib/websocket.js
index cd8929bdc..c157cfd2c 100644
--- a/lib/websocket.js
+++ b/lib/websocket.js
@@ -141,6 +141,7 @@ class WebSocket extends EventEmitter {
     const receiver = new Receiver(
       this._binaryType,
       this._extensions,
+      this._isServer,
       maxPayload
     );
 
diff --git a/test/receiver.test.js b/test/receiver.test.js
index 7d00c76cc..a70cc8dbe 100644
--- a/test/receiver.test.js
+++ b/test/receiver.test.js
@@ -48,7 +48,7 @@ describe('Receiver', () => {
   });
 
   it('parses a masked text message', (done) => {
-    const receiver = new Receiver();
+    const receiver = new Receiver(undefined, {}, true);
 
     receiver.on('message', (data) => {
       assert.strictEqual(data, '5:::{"name":"echo"}');
@@ -61,7 +61,7 @@ describe('Receiver', () => {
   });
 
   it('parses a masked text message longer than 125 B', (done) => {
-    const receiver = new Receiver();
+    const receiver = new Receiver(undefined, {}, true);
     const msg = 'A'.repeat(200);
 
     const list = Sender.frame(Buffer.from(msg), {
@@ -84,7 +84,7 @@ describe('Receiver', () => {
   });
 
   it('parses a really long masked text message', (done) => {
-    const receiver = new Receiver();
+    const receiver = new Receiver(undefined, {}, true);
     const msg = 'A'.repeat(64 * 1024);
 
     const list = Sender.frame(Buffer.from(msg), {
@@ -106,7 +106,7 @@ describe('Receiver', () => {
   });
 
   it('parses a 300 B fragmented masked text message', (done) => {
-    const receiver = new Receiver();
+    const receiver = new Receiver(undefined, {}, true);
     const msg = 'A'.repeat(300);
 
     const fragment1 = msg.substr(0, 150);
@@ -139,7 +139,7 @@ describe('Receiver', () => {
   });
 
   it('parses a ping message', (done) => {
-    const receiver = new Receiver();
+    const receiver = new Receiver(undefined, {}, true);
     const msg = 'Hello';
 
     const list = Sender.frame(Buffer.from(msg), {
@@ -172,7 +172,7 @@ describe('Receiver', () => {
   });
 
   it('parses a 300 B fragmented masked text message with a ping in the middle (1/2)', (done) => {
-    const receiver = new Receiver();
+    const receiver = new Receiver(undefined, {}, true);
     const msg = 'A'.repeat(300);
     const pingMessage = 'Hello';
 
@@ -221,7 +221,7 @@ describe('Receiver', () => {
   });
 
   it('parses a 300 B fragmented masked text message with a ping in the middle (2/2)', (done) => {
-    const receiver = new Receiver();
+    const receiver = new Receiver(undefined, {}, true);
     const msg = 'A'.repeat(300);
     const pingMessage = 'Hello';
 
@@ -280,7 +280,7 @@ describe('Receiver', () => {
   });
 
   it('parses a 100 B masked binary message', (done) => {
-    const receiver = new Receiver();
+    const receiver = new Receiver(undefined, {}, true);
     const msg = crypto.randomBytes(100);
 
     const list = Sender.frame(msg, {
@@ -302,7 +302,7 @@ describe('Receiver', () => {
   });
 
   it('parses a 256 B masked binary message', (done) => {
-    const receiver = new Receiver();
+    const receiver = new Receiver(undefined, {}, true);
     const msg = crypto.randomBytes(256);
 
     const list = Sender.frame(msg, {
@@ -324,7 +324,7 @@ describe('Receiver', () => {
   });
 
   it('parses a 200 KiB masked binary message', (done) => {
-    const receiver = new Receiver();
+    const receiver = new Receiver(undefined, {}, true);
     const msg = crypto.randomBytes(200 * 1024);
 
     const list = Sender.frame(msg, {
@@ -439,7 +439,7 @@ describe('Receiver', () => {
   });
 
   it('resets `totalPayloadLength` only on final frame (unfragmented)', (done) => {
-    const receiver = new Receiver(undefined, {}, 10);
+    const receiver = new Receiver(undefined, {}, false, 10);
 
     receiver.on('message', (data) => {
       assert.strictEqual(receiver._totalPayloadLength, 0);
@@ -452,7 +452,7 @@ describe('Receiver', () => {
   });
 
   it('resets `totalPayloadLength` only on final frame (fragmented)', (done) => {
-    const receiver = new Receiver(undefined, {}, 10);
+    const receiver = new Receiver(undefined, {}, false, 10);
 
     receiver.on('message', (data) => {
       assert.strictEqual(receiver._totalPayloadLength, 0);
@@ -467,7 +467,7 @@ describe('Receiver', () => {
   });
 
   it('resets `totalPayloadLength` only on final frame (fragmented + ping)', (done) => {
-    const receiver = new Receiver(undefined, {}, 10);
+    const receiver = new Receiver(undefined, {}, false, 10);
     let data;
 
     receiver.on('ping', (buf) => {
@@ -680,6 +680,40 @@ describe('Receiver', () => {
     receiver.write(Buffer.from([0x09, 0x00]));
   });
 
+  it('emits an error if a frame has the MASK bit off (server mode)', (done) => {
+    const receiver = new Receiver(undefined, {}, true);
+
+    receiver.on('error', (err) => {
+      assert.ok(err instanceof RangeError);
+      assert.strictEqual(
+        err.message,
+        'Invalid WebSocket frame: MASK must be set'
+      );
+      assert.strictEqual(err[kStatusCode], 1002);
+      done();
+    });
+
+    receiver.write(Buffer.from([0x81, 0x02, 0x68, 0x69]));
+  });
+
+  it('emits an error if a frame has the MASK bit on (client mode)', (done) => {
+    const receiver = new Receiver(undefined, {}, false);
+
+    receiver.on('error', (err) => {
+      assert.ok(err instanceof RangeError);
+      assert.strictEqual(
+        err.message,
+        'Invalid WebSocket frame: MASK must be clear'
+      );
+      assert.strictEqual(err[kStatusCode], 1002);
+      done();
+    });
+
+    receiver.write(
+      Buffer.from([0x81, 0x82, 0x56, 0x3a, 0xac, 0x80, 0x3e, 0x53])
+    );
+  });
+
   it('emits an error if a control frame has a payload bigger than 125 B', (done) => {
     const receiver = new Receiver();
 
@@ -811,7 +845,7 @@ describe('Receiver', () => {
   });
 
   it('emits an error if a frame payload length is bigger than `maxPayload`', (done) => {
-    const receiver = new Receiver(undefined, {}, 20 * 1024);
+    const receiver = new Receiver(undefined, {}, true, 20 * 1024);
     const msg = crypto.randomBytes(200 * 1024);
 
     const list = Sender.frame(msg, {
@@ -843,6 +877,7 @@ describe('Receiver', () => {
       {
         'permessage-deflate': perMessageDeflate
       },
+      false,
       25
     );
     const buf = Buffer.from('A'.repeat(50));
@@ -871,6 +906,7 @@ describe('Receiver', () => {
       {
         'permessage-deflate': perMessageDeflate
       },
+      false,
       25
     );
     const buf = Buffer.from('A'.repeat(15));
diff --git a/test/websocket-server.test.js b/test/websocket-server.test.js
index b9c460ce4..7503010a2 100644
--- a/test/websocket-server.test.js
+++ b/test/websocket-server.test.js
@@ -9,6 +9,7 @@ const http = require('http');
 const net = require('net');
 const fs = require('fs');
 
+const Sender = require('../lib/sender');
 const WebSocket = require('..');
 
 describe('WebSocketServer', () => {
@@ -744,7 +745,15 @@ describe('WebSocketServer', () => {
           }
         });
 
-        req.write(Buffer.from([0x81, 0x05, 0x48, 0x65, 0x6c, 0x6c, 0x6f]));
+        const list = Sender.frame(Buffer.from('Hello'), {
+          fin: true,
+          rsv1: false,
+          opcode: 0x01,
+          mask: true,
+          readOnly: false
+        });
+
+        req.write(Buffer.concat(list));
         req.end();
       });
 
diff --git a/test/websocket.test.js b/test/websocket.test.js
index b5b265248..229b78715 100644
--- a/test/websocket.test.js
+++ b/test/websocket.test.js
@@ -1293,7 +1293,7 @@ describe('WebSocket', () => {
       });
     });
 
-    it('can send text data with `mask` option set to `false`', (done) => {
+    it('honors the `mask` option', (done) => {
       const wss = new WebSocket.Server({ port: 0 }, () => {
         const ws = new WebSocket(`ws://localhost:${wss.address().port}`);
 
@@ -1301,30 +1301,29 @@ describe('WebSocket', () => {
       });
 
       wss.on('connection', (ws) => {
-        ws.on('message', (message) => {
-          assert.strictEqual(message, 'hi');
-          wss.close(done);
-        });
-      });
-    });
-
-    it('can send binary data with `mask` option set to `false`', (done) => {
-      const array = new Float32Array(5);
+        const chunks = [];
 
-      for (let i = 0; i < array.length; ++i) {
-        array[i] = i / 2;
-      }
-
-      const wss = new WebSocket.Server({ port: 0 }, () => {
-        const ws = new WebSocket(`ws://localhost:${wss.address().port}`);
+        ws._socket.prependListener('data', (chunk) => {
+          chunks.push(chunk);
+        });
 
-        ws.on('open', () => ws.send(array, { mask: false }));
-      });
+        ws.on('error', (err) => {
+          assert.ok(err instanceof RangeError);
+          assert.strictEqual(
+            err.message,
+            'Invalid WebSocket frame: MASK must be set'
+          );
+          assert.ok(
+            Buffer.concat(chunks)
+              .slice(0, 2)
+              .equals(Buffer.from('8102', 'hex'))
+          );
 
-      wss.on('connection', (ws) => {
-        ws.on('message', (message) => {
-          assert.ok(message.equals(Buffer.from(array.buffer)));
-          wss.close(done);
+          ws.on('close', (code, reason) => {
+            assert.strictEqual(code, 1002);
+            assert.strictEqual(reason, '');
+            wss.close(done);
+          });
         });
       });
     });