diff --git a/direct/src/leveleditor/ProtoPaletteBase.py b/direct/src/leveleditor/ProtoPaletteBase.py
index 7a9bb79d8b..381ee42806 100755
--- a/direct/src/leveleditor/ProtoPaletteBase.py
+++ b/direct/src/leveleditor/ProtoPaletteBase.py
@@ -10,6 +10,8 @@ from ObjectPaletteBase import *
 class ProtoPaletteBase(ObjectPaletteBase):
     def __init__(self):
         ObjectPaletteBase.__init__(self)
+        # self.dirname should be defined in inherited class
+        assert self.dirname
 
     def addItems(self):
         if type(protoData) == types.DictType: