Index: /trunk/src/VBox/Main/GuestImpl.cpp
===================================================================
--- /trunk/src/VBox/Main/GuestImpl.cpp	(revision 29637)
+++ /trunk/src/VBox/Main/GuestImpl.cpp	(revision 29638)
@@ -779,6 +779,9 @@
     CheckComArgStrNotEmptyOrNull(aCommand);
     CheckComArgOutPointerValid(aPID);
-    CheckComArgStrNotEmptyOrNull(aUserName); /* Do not allow anonymous executions (with system rights). */
     CheckComArgOutPointerValid(aProgress);
+
+    /* Do not allow anonymous executions (with system rights). */
+    if (RT_UNLIKELY((aUserName) == NULL || *(aUserName) == '\0'))
+        return setError(E_INVALIDARG, tr("No user name specified"));
 
     AutoCaller autoCaller(this);
@@ -1046,5 +1049,5 @@
                 {
                     rc = setError(VBOX_E_IPRT_ERROR,
-                                  tr("The operation was canceled."));
+                                  tr("The operation was canceled"));
                 }
             }