Why does a=g*sin(theta) on an inclined plane

I understand the derivation that on an inclined plane of angle θ, the acceleration of the object on the plane, parallel to the plane, is a=g*sinθ. However, I was just thinking about it, and should it not be a=g/sinθ ?

I got this because sinθ = g/a. θ is the angle between the ramp and the earth, g is the acceleration straight downwards (the opposite side from θ) and a is the hypotenuse of the triangle.
Rearranging, a=g/sinθ.

I feel like something must be wrong, but I can't see what it is.