diff --git a/setup.py b/setup.py
index 5b13ae2..3fcf225 100644
--- a/setup.py
+++ b/setup.py
@@ -33,7 +33,7 @@
         'setuptools',
         'Acquisition',
         'DateTime',
-        'ExtensionClass',
+        'ExtensionClass>=4.1a1',
         'Missing',
         'Persistence',
         'Record',