Two taylor usages --- can't feel the difference.

I don't understand why the first taylor doesn't work, while the second one works:

kill(all)$

depends([a,b], [x, t]);

taylor(f(a,b), [a,b], [aa, bb], 2);

taylor(f(a,b), a, aa, 2, b, bb, 2);

For the first I get

Attempt to differentiate with respect to a number:
0
 -- an error. To debug this try: debugmode(true);


Both of them work if there is no depends([a,b], [x, t])