--- a/mercurial/help.py	Fri Mar 03 13:32:10 2017 -0500
+++ b/mercurial/help.py	Wed Mar 08 18:31:33 2017 -0500
@@ -618,6 +618,8 @@
     """
     if keep is None:
         keep = []
+    else:
+        keep = list(keep) # make a copy so we can mutate this later
     fullname = name
     section = None
     subtopic = None