We have observed that the terahertz power emitted by (100) n-type InP exhibits a single maximum and a single minimum as the crystal is rotated through 360° about its surface normal. This stands in contrast to other semiconductor terahertz emitters for which two, three, or four maxima per rotation have been observed. We have investigated the terahertz emission as a function of sample doping, optical excitation fluence, and applied in-plane magnetic field. The data cannot be accounted for by bulk optical rectification. We suggest that the origin of the phenomenon may be related to crystal twinning.