diff --git a/product/ERP5Type/tests/testDynamicClassGeneration.py b/product/ERP5Type/tests/testDynamicClassGeneration.py
index f2fe59366139c6ec26fb2520d281a4badb37aaa2..35720b966728d79ae1b7679fb4b6e0cb4036c220 100644
--- a/product/ERP5Type/tests/testDynamicClassGeneration.py
+++ b/product/ERP5Type/tests/testDynamicClassGeneration.py
@@ -1186,7 +1186,7 @@ class TestZodbImportFilesystemPropertySheet(ERP5TypeTestCase):
     from Products.ERP5Type import PropertySheet
     # Get all the property sheets defined on the filesystem
     for name, klass in PropertySheet.__dict__.iteritems():
-      if name[0] == '_':
+      if name[0] == '_' or isinstance(klass, basestring):
         continue
       filesystem_property_sheet = klass
       property_sheet_name = name