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Sorry, I cannot reproduce your error on SageMathCloud: it works for me, cf.
https://cloud.sagemath.com/projects/551a1e1d-9360-47bf-89ba-91603e96c7fe/files/test_coordinates.sagews Could it be that you typed the second example in the same worksheet as the first one? In this case, the error arises because one cannot have the symbolic variable theta both in (0,2pi) and (-pi,pi).