I do a fair amount of scripting with Quark (up through ver. 4.11) but I'm not familiar with an application term carrierTab. Is this an application term in ver. 5.x? which I assume since you are looking at tables) or is it a variable you've defined?
Another possiblility is "myVar as String". It may be that carrierTab has a special data type (like color spec does) that won't coerce to a string. Do you need it as a string? Or do you need its contents, as a string?
Have you tried setting appleScript delimiters to tab and getting "first text item" as an alternate approach?
Since I'm not too sure what you need to accomplish, I don't know if these suggestions are helpful. You may find more help if you include more script information, either here, on the Quark Scripting Forum, or on the AppleScript Users List where these questions show up regularly.