Index: /trunk/src/VBox/Frontends/VBoxManage/VBoxManageGuestProp.cpp
===================================================================
--- /trunk/src/VBox/Frontends/VBoxManage/VBoxManageGuestProp.cpp	(revision 24023)
+++ /trunk/src/VBox/Frontends/VBoxManage/VBoxManageGuestProp.cpp	(revision 24024)
@@ -310,5 +310,5 @@
 
         if (!pszValue && !pszFlags)
-            CHECK_ERROR(machine, SetGuestPropertyValue(Bstr(pszName), Bstr()));
+            CHECK_ERROR(machine, SetGuestPropertyValue(Bstr(pszName), Bstr("")));
         else if (!pszFlags)
             CHECK_ERROR(machine, SetGuestPropertyValue(Bstr(pszName), Bstr(pszValue)));
Index: /trunk/src/VBox/Main/MachineImpl.cpp
===================================================================
--- /trunk/src/VBox/Main/MachineImpl.cpp	(revision 24023)
+++ /trunk/src/VBox/Main/MachineImpl.cpp	(revision 24024)
@@ -3363,7 +3363,7 @@
                 rc = E_FAIL;
             else
-                rc = directControl->AccessGuestProperty(aName, aValue, aFlags,
-                                                        true /* isSetter */,
-                                                        &dummy, &dummy64, &dummy);
+                rc = directControl->AccessGuestProperty
+                             (aName, *aValue ? aValue : NULL, aFlags,
+                              true /* isSetter */, &dummy, &dummy64, &dummy);
         }
     }