diff --git a/Lib/imputil.py b/Lib/imputil.py
index 8b37ef4..a907287 100644
--- a/Lib/imputil.py
+++ b/Lib/imputil.py
@@ -51,7 +51,7 @@
         self.namespace['__import__'] = self.previous_importer
 
     def add_suffix(self, suffix, importFunc):
-        assert callable(importFunc)
+        assert hasattr(importFunc, '__call__')
         self.fs_imp.add_suffix(suffix, importFunc)
 
     ######################################################################
@@ -539,7 +539,7 @@
         self.suffixes = [ ]
 
     def add_suffix(self, suffix, importFunc):
-        assert callable(importFunc)
+        assert hasattr(importFunc, '__call__')
         self.suffixes.append((suffix, importFunc))
 
     def import_from_dir(self, dir, fqname):