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"For does not localize the iterator i" -- that alone for me is a sufficient argument to avoid For. All the rest are good points, but mostly about style / readability whereas this one actually breaks the functional pattern (I strive to have as few assignments as possible in general) and may lead to hard-to-find bugs (as opposed to simple typo's that Mathematica will tell you about).
The answer about adding PrincipalValue also makes me suspect that the problem is a is treated as a complex number. So rather than a > 0, ElementQ[a, Reals] is probably sufficient :)
