diff --git a/lib/websocket-server.js b/lib/websocket-server.js
index db02f4d09..b74eb1cfa 100644
--- a/lib/websocket-server.js
+++ b/lib/websocket-server.js
@@ -186,12 +186,14 @@ class WebSocketServer extends EventEmitter {
       req.headers['sec-websocket-key'] !== undefined
         ? req.headers['sec-websocket-key'].trim()
         : false;
+    const upgrade = req.headers.upgrade;
     const version = +req.headers['sec-websocket-version'];
     const extensions = {};
 
     if (
       req.method !== 'GET' ||
-      req.headers.upgrade.toLowerCase() !== 'websocket' ||
+      upgrade === undefined ||
+      upgrade.toLowerCase() !== 'websocket' ||
       !key ||
       !keyRegex.test(key) ||
       (version !== 8 && version !== 13) ||
diff --git a/test/websocket-server.test.js b/test/websocket-server.test.js
index 5d152ab73..391b09e9d 100644
--- a/test/websocket-server.test.js
+++ b/test/websocket-server.test.js
@@ -383,6 +383,47 @@ describe('WebSocketServer', () => {
   });
 
   describe('Connection establishing', () => {
+    it('fails if the Upgrade header field value cannot be read', (done) => {
+      const server = http.createServer();
+      const wss = new WebSocket.Server({ noServer: true });
+
+      server.maxHeadersCount = 1;
+
+      server.on('upgrade', (req, socket, head) => {
+        assert.deepStrictEqual(req.headers, { foo: 'bar' });
+        wss.handleUpgrade(req, socket, head, () => {
+          done(new Error('Unexpected callback invocation'));
+        });
+      });
+
+      server.listen(() => {
+        const req = http.get({
+          port: server.address().port,
+          headers: {
+            foo: 'bar',
+            bar: 'baz',
+            Connection: 'Upgrade',
+            Upgrade: 'websocket'
+          }
+        });
+
+        req.on('response', (res) => {
+          assert.strictEqual(res.statusCode, 400);
+
+          const chunks = [];
+
+          res.on('data', (chunk) => {
+            chunks.push(chunk);
+          });
+
+          res.on('end', () => {
+            assert.strictEqual(Buffer.concat(chunks).toString(), 'Bad Request');
+            server.close(done);
+          });
+        });
+      });
+    });
+
     it('fails if the Sec-WebSocket-Key header is invalid (1/2)', (done) => {
       const wss = new WebSocket.Server({ port: 0 }, () => {
         const req = http.get({