can't set spot color to end of list

The following script will get a list of every fill color used in an InDesign CS document.
It works if all the colors are process. But if it encounters a spot color I get a message on line 17, something like:

Can’t set end of “PANTONE 3415 C” to “Black
C=0 M=100 Y=0 K=0
C=100 M=90 Y=10 K=0”.

The answer’s probably simple, but it’s really hanging me up.

set theFillList to {}
tell application "InDesign CS"
	set myDoc to document 1
	tell myDoc
		set everyFrame to (every text frame)
		repeat with T in everyFrame
			repeat with i from 1 to (count everyFrame)
				set TheFrame to (item i of everyFrame)
				set all_fills to (the name of fill color of every word of TheFrame) as list
				set return_list to {}
				repeat with a_fill in all_fills
					if return_list does not contain a_fill then set end of return_list to (contents of a_fill)
				end repeat
				set this_fill to return_list
				set {ATID, AppleScript's text item delimiters} to {AppleScript's text item delimiters, return}
				set this_fill to this_fill as string
				if theFillList does not contain {(this_fill)} then set end of (theFillList) to (this_fill)
				set AppleScript's text item delimiters to ATID
				set theFillList to theFillList as string --this gets every fill color of page
			end repeat
		end repeat
	end tell
end tell

Okay, I figured it out. Those two mischievous little words on line 19 “as string”.