Wigner's effective mathematics and contradiction

Complex numbers are basic. An inconsistency would question Wigner's unreasonable effectiveness of mathematics. A vehicle to study this question is Kirchoff's scalar diffraction theory. In the paper, an inconsistency in complex phase angle is presented. When this inconsistency is introduced in Kirchoff's theory we can study its influence on the experimental success of this theory. There are no \emph{a priori} reasons to include or exclude phase angles. Referring to Wigner, an experiment can provide more insight. In the experiment a weak intensity, small wavelength source can be employed. When the contradictory phase angle is excluded, a nonzero diffraction amplitude appears physically possible. If it is included, this amplitude vanishes.