Sweetners are not necessarily calorie free. Things like Xylitol and Innositol are commonly used sweetners in gum and sugarfree mints (and are also laxatives!) but do contain calories.
I'll continue chewing, I don't think a syn or two is going to derail my diet. Isn't that why they made stock free? Because the syn value was so small it couldn't possibly add up any significant difference?