Raising an arb number with 0 midpoint but radius gives nan

[zacmon]

If I have a=arb("[+/- 7.53e-121]"), raising this to any power greater than 0, e.g., a**1 gives nan. On the other hand, having b = arb(0) and raising this to a power greater than 0, e.g. b**1, gives 0. Why does the former, which has a midpoint of 0 but with nonzero radius, return nan when raised to a power, but the latter, lacking a radius but with midpoint 0, gives 0?

I am using python-flint 0.30, Arb 2.17, Flint 2.5.2, MacOS 10.13.6.