What's wrong (Why isn't Maple responding 'True')?

restart;

assume(n, natural);

evalb(sum(binomial(n, k)*x^(n-k)*y^k, k = 0 .. n) = (y+x)^n);

false

simplify(sum(binomial(n, k)*x^(n-k)*y^k, k = 0 .. n));

(y+x)^n~

The last line shows it's supposed to be true... it's the binomial theorem. Is there something i'm missing?