I am unable to get a Filemaker record by ID using AppleScript

I am scripting FileMaker pro in Applescript - all was going great until I hit a wall. I can get records from any table matching any field against its value EXCEPT for the ID - which is a bit of a deal breaker. I have tried dozens of ways and they all fail - generally with an error -1728.

This is example code:

on getRawDBDRowTest(DB_DEF, _id)
	local rowCount, rowID, i, allRaw
	try
		tell application "FileMaker Pro"
			activate
			tell database the (dbName of DB_DEF)
				tell table the (tableName of DB_DEF)
					go to layout the (tableName of DB_DEF)
					set rowCount to the count of records
					if (rowCount > 0) then
						set rec to every record whose cell "taskID" = _id
						return rec
					else
						return {}
					end if
				end tell
			end tell
		end tell
	on error errTxt number errNum
		if errNum is equal to -1728 then
			--No results
			return {}
		else
			error "DBUtils/getRawDBDRow
" & errTxt
		end if
	end try
end getRawDBDRowTest

This is my field definition:

I can confirm that I have valid GUIDS in my data:

I have lost a full working day on what should be a simple thing - any pointers?