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Indeed this is an old issue: I.parent() should not be the symbolic ring [trac ticket #18036]. When you know your base field, you should use the $i$ from that base field. You can access it e.g. by CDF(I) or CDF.gens()[0] or CDF.0. If you have a complex number z which is not in your base field for whatever reason (e.g. it is symbolic, e.g. because you used I), then you can do CDF(z) to convert it into your base field. You can also change the ring of a matrix e.g. by C.change_ring(CDF); it returns a new matrix, over the new ring.