I had the similar issue when i was trying to assign a list value to the project name field filter. I was using the following code when i got this error:
BugFilter.Filter("BG_PROJECT") = myList.RootNode.Children(icounter).Name
I figured that if the assigned value is wrapped around in double quotes then the issue was fixed. So i modified the code as given below and the problem was fixed:
BugFilter.Filter("BG_PROJECT") = Chr(34) & myList.RootNode.Children(icounter).Name & Chr(34)
Hope this would help anyone else searching for the same problem.