Automator (Applescript) and Pages: "could not be interpreted as a file URL."

I have the following applescript as the sole component of an Automator app:

on run {input, parameters}
	repeat with theFile in input
		tell application "Pages"
			set theDoc to open theFile
			set theDocName to name of theDoc
            -- I implemented the sanitization of spaces to underscores in hope of fixing this present issue but it has not helped.
			set sanitizedDocName to do shell script "echo " & quoted form of theDocName & " | sed -e 's/ /_/g'"
			set theName to sanitizedDocName & "-auto-text"
			display dialog (":Users:slu:automator-text-export:" & theName & ".txt")
			export front document to file (":Users:slu:automator-text-export:" & theName & ".txt") as unformatted text
			close theDoc saving no
		end tell
	end repeat
	return input
end run

I think the intention is self-explanatory. I can use the snippets I already found online to ask finder for the path the input file was from, e.g.:

tell application "Finder"
            -- Actually theFilesFolder is unused here
			set theFilesFolder to (folder of theFile) as text
		end tell

This snippet works to successfully export to a place relative to the files' original positions. But I specifically want to make all my exports go into a predermined hardcoded directory, but I'm failing here on where I can find documentation on how to build a proper AppleScript representation of a filesystem location. I've also tried assembling the path with just slashes instead of colons but it makes for a different kind of error ("Could not be exported as ").