Question transform.Rotate not working as intended

This is probably some simple error or maybe I'm not understanding Vectors correctly. Currently I'm trying to create pushable object that the player can walk up to then grab and rotate or push it. Rotating works fine until the cube is flipped on its side. Here is the code I'm using to rotate it. (This object has a rigidbody if that effects anything)

Code (CSharp):

1.             pushDirection = horizontalInput * rotationSpeed * Time.fixedDeltaTime;
2.             transform.Rotate(Vector3.up, pushDirection);

I believe the cube is trying to use its local up-vector. But shouldn't Vector3.up be using the world up-vector? I tried to switch Vector3.up with transform.up but it gives the same results. Anyone see what I'm doing wrong?

 

If you look at the documentation, you can see Transform.Rotate has an overload which tells it to either use local or global space, and the default is local space.