diff --git a/reactos/dll/win32/kernel32/winnls/string/nls.c b/reactos/dll/win32/kernel32/winnls/string/nls.c
index 4c0c3256495..1b1013a2af0 100644
--- a/reactos/dll/win32/kernel32/winnls/string/nls.c
+++ b/reactos/dll/win32/kernel32/winnls/string/nls.c
@@ -755,6 +755,12 @@ IntWideCharToMultiByteUTF8(UINT CodePage,
     INT TempLength;
     DWORD Char;
 
+    if (Flags)
+    {
+        SetLastError(ERROR_INVALID_FLAGS);
+        return 0;
+    }
+
     /* Does caller query for output buffer size? */
     if (MultiByteCount == 0)
     {