Help with apple script

[nicemonkey]

Help, I need some trouble shooting advice with apple script, I have set up a script to convert a jpeg and eps into one zip file but come up with an error. which is below

Can�t make �class cfol� "zip folder" of �class cfol� "Desktop" of �class cfol� "michaeltravers" of �class cfol� "Users" of �class sdsk� of application "Finder" into the expected type.

This is the script

tell application "Finder"
set startFolder to (folder "Macintosh HD:Users:michaeltravers:Desktop:zip folder")
set startFolder_POSIX to quoted form of POSIX path of startFolder -- we'll need this later

-- get the JPEG filenames
set JPEGfiles to (name of every file of startFolder whose name extension is "jpeg") as alias list
-- iterate through them
repeat with eachFile in JPEGfiles
-- get the base name (without the extension)
set baseName to (characters 1 through -6 of eachFile) as text
-- do we have a corresponding .eps file?
if exists file (baseName & ".eps") of folder startFolder then
-- if so, create a zip file containing the two items
set zipfilename to quoted form of (baseName & ".zip")
set jpegfilename to quoted form of (baseName & ".jpeg")
set epsfilename to quoted form of (baseName & ".eps")
do shell script "cd " & startFolder_POSIX & "; zip " & zipfilename & space & jpegfilename & space & epsfilename
end if
end repeat

end tell

any advice on where I am going wrong would be gratefully received.