Comment 2 for bug 266135

I had the same issue after upgrading to 2.1.5....
This can be caused when you have a corrupted list or a list
does not contain all of it's pieces.

I have a small number of lists, so I ran the list_members script
for each list until I found the offending list. In my case it was
easy because it was a dummy "test" list that I could remove.

If others have this issue on a valid list, then they will need to
backup the lists files and recreate it. (someone else may have
a beter way to fix the corruption depending upon what it is)