AppleScript error on Tiger only: "can't make file into type alias"

Hi,

I'm successfully using this smart TextWrangler applescript from this webpage for opening files on the same folder as the current document:

http://blog.ampli.fi/textwrangler-open-file-from-directory-of-current-file/

It works fine on Mountain Lion.

However, when I try it on Tiger 10.4.11, this error dialog appears: "TextWrangler got an error: Can't make [file] into type alias".

The script basically gets the path of the current document in TextWrangler and posts a file chooser on such path.

I guess these are the lines that Tiger doesn't like:

setcurrent_pathtoPOSIX pathofcurrent_file

setnew_filetochoose filedefault location (current_path) withmultiple selections allowed

opennew_file

After googling for the error "can't make [] into type alias", I tried to change such lines to:

setcurrent_pathto (POSIX pathof (current_file as alias))

setnew_filetochoose filedefault location (current_path) withmultiple selections allowed

opennew_file

But, however, the same error remains.

Do you have any idea on how could this work?

Thanks!