Body cap and rear lens cap for NEX 6...

I understand that a body cap or a rear lens cap are included in the NEX 6L kit - body and 16-50 lens.  Normally that is not an issue - when I put another lens on, I simply take the rear lens cap from that lens and put it on the one I am removing.

I just purchased a Rokinon 8mm fisheye for the Nex.  Lens seems fine, but the rear cap does not fit any of my Sony lenses very well.  I can get it on after working with it for a while - but the rear lens cap that is with my Sigma 19mm lens works fine - just like the Sony rear lens caps.

Since the lens fits properly with no issues, they obviously have reverse-engineered the e-mount properly.  Has anyone else had an issue with rear lens caps from third-party lenses not fitting properly on Sony lenses?  (I have also noticed this issue on a few third-party lenses on my Nikon system, but not to the extent of this situation.)

Just trying to find out if this is normal, or I possibly have a cap that is out of spec.

Thanks...