2d sprite falls through floor

Hey so I'm pretty new to unity and have done the roll a ball, space shooter, and seen a few 2d tutorials. Anyway, I wanted to try starting a little 2d project. I have my sprite set up with a 2d rigid body component, and then have it surrounded by a 2d box collider. I put a cube out and stretched it, then attached a 2d edge collider so it would act as a floor. I have then both in the same z value position, but if you press play, the sprite just falls through the cube. I've looked around and couldn't find any definitive answers. Any suggestions would be awesome.