What am I doing wrong in this integration problem?

Hi, I've been working on this integration problem, but when I upload it to our homework program the computer keeps saying that it's wrong. It's:

$\displaystyle \int \frac{z^5}{\sqrt[3]{9+z^6}} du$

So what I did was:

$\displaystyle u = 9 + z^6$

$\displaystyle \frac{du}{6} = z^5 dz$

$\displaystyle \frac{1}{6}\int u^{\frac{-1}{3}} du$

$\displaystyle \frac{-1}{18 \sqrt[3]{(9+z^6)^4}}$

Do you think it's a syntax problem, or am I making an obvious mistake?