diff --git a/mercurial/extensions.py b/mercurial/extensions.py
--- a/mercurial/extensions.py
+++ b/mercurial/extensions.py
@@ -674,9 +674,8 @@ def wrapfunction(container, funcname, wr
     assert callable(wrapper)
 
     if not isinstance(funcname, str):
-        msg = b"pass wrapfunction target name as `str`, not `bytes`"
-        util.nouideprecwarn(msg, b"6.6", stacklevel=2)
-        funcname = pycompat.sysstr(funcname)
+        msg = b"wrapfunction target name should be `str`, not `bytes`"
+        raise TypeError(msg)
 
     origfn = getattr(container, funcname)
     assert callable(origfn)