text file problem

I am writing a simple script to ‘fix’ subtitle files to work on my DVD player. Pretty simple, my problem occurs while replacing html via sed:

set thisText to (do shell script "sed -e 's/<[^>]*>/poo/g' " & quoted form of POSIX path of (this_item)) as Unicode text

This works but when I diff the files I am told there is a nul character at offset 0 so something is wonky. I can open the file in textedit and can tell the difference. I guess I need to not have that. When writing I tried setting the eof to 1 and the error went to offset 1. I hoped using as Unicode text was the answer but it is not.

Thank you

Model: imac g3
AppleScript: 1.10.7
Browser: Firefox 3.5.5
Operating System: Mac OS X (10.4)