diff --git a/Lib/fontTools/subset/util.py b/Lib/fontTools/subset/util.py
index d20e925d7..7be1a22de 100644
--- a/Lib/fontTools/subset/util.py
+++ b/Lib/fontTools/subset/util.py
@@ -12,10 +12,8 @@ def _add_method(*clazzes):
                 continue  # Support multiple names of a clazz
             done.append(clazz)
             assert clazz.__name__ != "DefaultTable", "Oops, table class not found."
-            assert not hasattr(
-                clazz, method.__name__
-            ), "Oops, class '%s' has method '%s'." % (clazz.__name__, method.__name__)
-            setattr(clazz, method.__name__, method)
+            if not hasattr(clazz, method.__name__):
+                setattr(clazz, method.__name__, method)
         return None
 
     return wrapper