The $tolist var will confuse you.
ListMail reads and then removes the "rs" and "set" substrings when interpreting the value. With "rs" the ID is not a list at all, but rather a User Selection set ID.
It does, however, appear that for some reason ListMail -is- trying to set the int field to contain a string that should not be there.
Are you on the latest files available from the member area? If so, please submit your information
here along with a link back to this post and exact steps to recreate this consistently.
Thanks