diff --git a/src/msw/filedlg.cpp b/src/msw/filedlg.cpp
index 63e5f18dfd..ef9fd75fd8 100644
--- a/src/msw/filedlg.cpp
+++ b/src/msw/filedlg.cpp
@@ -425,7 +425,7 @@ int wxFileDialog::ShowModal()
         defextBuffer = AppendExtension(wxT("a"), extension);
         if (defextBuffer.StartsWith(wxT("a.")))
         {
-            defextBuffer.Mid(2);
+            defextBuffer = defextBuffer.Mid(2); // remove "a."
             of.lpstrDefExt = defextBuffer.c_str();
         }
     }