diff --git a/smack-tcp/src/main/java/org/jivesoftware/smack/tcp/XMPPTCPConnection.java b/smack-tcp/src/main/java/org/jivesoftware/smack/tcp/XMPPTCPConnection.java
index f29c8bcf1..1bca84067 100644
--- a/smack-tcp/src/main/java/org/jivesoftware/smack/tcp/XMPPTCPConnection.java
+++ b/smack-tcp/src/main/java/org/jivesoftware/smack/tcp/XMPPTCPConnection.java
@@ -254,7 +254,9 @@ public class XMPPTCPConnection extends AbstractXMPPConnection {
             throw new AlreadyLoggedInException();
         }
         // Do partial version of nameprep on the username.
-        username = username.toLowerCase(Locale.US).trim();
+        if (username != null) {
+            username = username.toLowerCase(Locale.US).trim();
+        }
 
         if (saslAuthentication.hasNonAnonymousAuthentication()) {
             // Authenticate using SASL