'File Usage Error'

I have some 25 files (all .tps) to selectively access and read from. I
use a Case structure to select the desired file with code like this:

Of 'L001'
Access:L001.Usefile()    ** This line gives me a 'File Usage
Error'
Set(L001.LblZipKey)
L001.LblZip = Tag:DestZip
If Access:L001.Fetch(L001.LblZipKey) <> Level:Benign
Beep(Beep:SystemHand)
Message(Error())
End
The files have all been selected for use in the Procedure Properties
window. I haven't tried all the files but all I've tried so far give
me the error.
I'm using 5.5e.
I use 'Access:....Usefile() frequently without problems. Any ideas on
what's going on? Thanks for any help!
Oren Glick