diff --git a/dlls/advpack/advpack_private.h b/dlls/advpack/advpack_private.h
index a1bcfe44eb0..5d7489778d6 100644
--- a/dlls/advpack/advpack_private.h
+++ b/dlls/advpack/advpack_private.h
@@ -44,7 +44,8 @@ static inline char *heap_strdupWtoA(const WCHAR *str)
     if(str) {
         size_t size = WideCharToMultiByte(CP_ACP, 0, str, -1, NULL, 0, NULL, NULL);
         ret = heap_alloc(size);
-        WideCharToMultiByte(CP_ACP, 0, str, -1, ret, size, NULL, NULL);
+        if(ret)
+            WideCharToMultiByte(CP_ACP, 0, str, -1, ret, size, NULL, NULL);
     }
 
     return ret;