How to find the interval of convergence for $\frac{8x^{n}}{9^{n+1}}$.

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I want to find the interval of convergence for $$\frac{8x^{n}}{9^{n+1}}.$$ I've tried everything and just can't seem to find it. I looked up a calculator and it said $(-9, 9)$, but I don't understand why it wouldn't be $[-9,9)$.

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Suppose $x=-9$. Then

$$\frac{8x^n}{9^{n+1}} = \frac{8(-9)^n}{9^{n+1}} = \frac{8(-1)^n9^n}{9^{n+1}}= \frac 8 9 \cdot (-1)^n$$

This obviously doesn't converge.