Zero has become special with this PR: previously, the assertion was correct since `c.x[0]`, which has units of kpc, is not equal to `0`, which does not have these units. However, with the present PR, `0` can have any units, and thus one does have `c.x[0] == 0`  

(I must admit that I'm not 100% sure that the above was the intent of the test...)