diff --git a/lib/websocket.js b/lib/websocket.js
index c39b34cdf..1802274b0 100644
--- a/lib/websocket.js
+++ b/lib/websocket.js
@@ -783,6 +783,11 @@ function initAsClient(websocket, address, protocols, options) {
 
     req = websocket._req = null;
 
+    if (res.headers.upgrade.toLowerCase() !== 'websocket') {
+      abortHandshake(websocket, socket, 'Invalid Upgrade header');
+      return;
+    }
+
     const digest = createHash('sha1')
       .update(key + GUID)
       .digest('base64');
diff --git a/test/websocket.test.js b/test/websocket.test.js
index 717ecbdf3..36d3ba0a6 100644
--- a/test/websocket.test.js
+++ b/test/websocket.test.js
@@ -525,6 +525,26 @@ describe('WebSocket', () => {
     beforeEach((done) => server.listen(0, done));
     afterEach((done) => server.close(done));
 
+    it('fails if the Upgrade header field value is not "websocket"', (done) => {
+      server.once('upgrade', (req, socket) => {
+        socket.on('end', socket.end);
+        socket.write(
+          'HTTP/1.1 101 Switching Protocols\r\n' +
+            'Connection: Upgrade\r\n' +
+            'Upgrade: foo\r\n' +
+            '\r\n'
+        );
+      });
+
+      const ws = new WebSocket(`ws://localhost:${server.address().port}`);
+
+      ws.on('error', (err) => {
+        assert.ok(err instanceof Error);
+        assert.strictEqual(err.message, 'Invalid Upgrade header');
+        done();
+      });
+    });
+
     it('fails if the Sec-WebSocket-Accept header is invalid', (done) => {
       server.once('upgrade', (req, socket) => {
         socket.on('end', socket.end);