Index: /trunk/src/VBox/HostServices/GuestProperties/service.cpp
===================================================================
--- /trunk/src/VBox/HostServices/GuestProperties/service.cpp	(revision 40360)
+++ /trunk/src/VBox/HostServices/GuestProperties/service.cpp	(revision 40361)
@@ -1148,5 +1148,5 @@
         /* Send out a host notification */
         if (RT_SUCCESS(rc))
-            rc = notifyHost(pszProperty, NULL, u64Timestamp, NULL);
+            rc = notifyHost(pszProperty, "", u64Timestamp, "");
     }
     LogFlowThisFunc(("returning\n"));
Index: /trunk/src/VBox/Main/src-server/MachineImpl.cpp
===================================================================
--- /trunk/src/VBox/Main/src-server/MachineImpl.cpp	(revision 40360)
+++ /trunk/src/VBox/Main/src-server/MachineImpl.cpp	(revision 40361)
@@ -5352,5 +5352,7 @@
             /** @todo r=bird: Why aren't we leaving the lock here?  The
              *                same code in PushGuestProperty does... */
-            mParent->onGuestPropertyChange(mData->mUuid, aName, aValue, aFlags);
+            mParent->onGuestPropertyChange(mData->mUuid, aName,
+                                           aValue ? aValue : Bstr("").raw(),
+                                           aFlags ? aFlags : Bstr("").raw());
         }
     }
@@ -11919,6 +11921,6 @@
 
     CheckComArgStrNotEmptyOrNull(aName);
-    CheckComArgMaybeNull(aValue);
-    CheckComArgMaybeNull(aFlags);
+    CheckComArgNotNull(aValue);
+    CheckComArgNotNull(aFlags);
 
     try