Index: /trunk/src/VBox/Main/NATEngineImpl.cpp
===================================================================
--- /trunk/src/VBox/Main/NATEngineImpl.cpp	(revision 34991)
+++ /trunk/src/VBox/Main/NATEngineImpl.cpp	(revision 34992)
@@ -287,10 +287,11 @@
     {
         r = it->second;
-        if (   it->first == name
-            || (   r.strHostIP == Utf8Str(aBindIp)
-                && r.u16HostPort == aHostPort))
-        {
-            return E_INVALIDARG;
-        }
+        if (it->first == name)
+            return setError(E_INVALIDARG,
+                            tr("A NAT rule of this name does already exist"));
+        if (   r.strHostIP == Utf8Str(aBindIp)
+            && r.u16HostPort == aHostPort)
+            return setError(E_INVALIDARG,
+                            tr("A NAT rule for this host port and this host IP does already exist"));
     }