diff --git a/client/src/main/java/ch/zhaw/pm2/multichat/client/ChatWindowController.java b/client/src/main/java/ch/zhaw/pm2/multichat/client/ChatWindowController.java
index 1a8a9c6..c657f44 100644
--- a/client/src/main/java/ch/zhaw/pm2/multichat/client/ChatWindowController.java
+++ b/client/src/main/java/ch/zhaw/pm2/multichat/client/ChatWindowController.java
@@ -106,18 +106,22 @@ public class ChatWindowController {
 
     private void startConnectionHandler() throws IOException {
         String userName = userNameField.getText();
-        String serverAddress = serverAddressField.getText();
-        int serverPort = Integer.parseInt(serverPortField.getText());
-        connectionHandler = new ClientConnectionHandler(
-            NetworkHandler.openConnection(serverAddress, serverPort), userName,
-            messages);
-        new Thread(connectionHandler).start();
+        if(!userName.contains(" ")) {
+            String serverAddress = serverAddressField.getText();
+            int serverPort = Integer.parseInt(serverPortField.getText());
+            connectionHandler = new ClientConnectionHandler(
+                    NetworkHandler.openConnection(serverAddress, serverPort), userName,
+                    messages);
+            new Thread(connectionHandler).start();
 
-        //register Listener
-        startListener();
+            //register Listener
+            startListener();
 
-        // register window close handler
-        rootPane.getScene().getWindow().addEventHandler(WindowEvent.WINDOW_CLOSE_REQUEST, windowCloseHandler);
+            // register window close handler
+            rootPane.getScene().getWindow().addEventHandler(WindowEvent.WINDOW_CLOSE_REQUEST, windowCloseHandler);
+        } else {
+            addError("It is not allowed to have spaces in username!");
+        }
     }
 
     private void terminateConnectionHandler() {