diff --git a/lib/modules/webserver/server.js b/lib/modules/webserver/server.js
index d10c9165..fcd0a83c 100644
--- a/lib/modules/webserver/server.js
+++ b/lib/modules/webserver/server.js
@@ -6,6 +6,8 @@ const express = require('express');
 const fs = require('../../core/fs');
 require('http-shutdown').extend();
 
+const WEB_SOCKET_STATE_OPEN = 1;
+
 class Server {
   constructor(options) {
     this.buildDir = options.buildDir;
@@ -31,15 +33,17 @@ class Server {
     const main = serveStatic(this.buildDir, {'index': ['index.html', 'index.htm']});
 
     this.app = express();
+    expressWebSocket(this.app);
+
     this.app.use(main);
     this.app.use('/coverage', coverage);
     this.app.use(coverageStyle);
 
-    expressWebSocket(this.app);
-
     this.app.ws('/', function(ws, _req) {
       self.events.on('outputDone', () => {
-        ws.send('outputDone');
+        if (ws.readyState === WEB_SOCKET_STATE_OPEN) {
+          ws.send('outputDone');
+        }
       });
     });