diff --git a/pandatool/src/egg-palettize/imageFile.cxx b/pandatool/src/egg-palettize/imageFile.cxx
index af583a17b7..b97014ce1e 100644
--- a/pandatool/src/egg-palettize/imageFile.cxx
+++ b/pandatool/src/egg-palettize/imageFile.cxx
@@ -209,7 +209,7 @@ set_filename(const string &dirname, const string &basename) {
   }
 
   if (_properties._alpha_type != (PNMFileType *)NULL) {
-    _alpha_filename = _filename.get_fullpath_wo_extension() + "_a";
+    _alpha_filename = _filename.get_fullpath_wo_extension() + "_a.";
     _alpha_filename.set_extension
       (_properties._alpha_type->get_suggested_extension());
   }