contrologix FSC

Hi there...I am having trouble with an FSC. I am programming on the contrologix 5000 and keep getting an invalid expression in my FSC. I want to use this instruction to look for a match in two files. The control is fine, the lenght of these files is 15, my position is 0, I am using incremental and the expression I want is "Error_codes = Error_scan_file".
These file have been created in my controller tags, and they are both a length of 15. I have tried using the following expressions for the FSC and all are being rejected when I verify the rung. It keeps saying "Invalid expression"

Error_scan_file=Error_codes
Error_scan_file = Error_codes
Error_scan_file[=]Error_codes
Error_scan_file [=] Error_codes

I am pulling the names right from the tag data base so I know the spelling matches the tags. Any help would be GREAT!!!!!!!!!!!
I am also wondering if the FSC uses the first position as a dummy one like other file instruction..meaning, do I need to use a length of 16 and start in position 1?

If both tags are arrays (don't have files as such in CLX), then you need to index them - most commonly with the .POS of the instruction's control tag.

Look at the examples in the RSLogix 5000 help file.