The performance of a spoken language system suffers when non-speech is incorrectly classified as speech. Singing is particularly difficult to discriminate from speech, since both are natural language. However, singing conveys a melody, whereas speech does not; in particular, a singer's fundamental frequency should not deviate significantly from an underlying sequence of notes, while a speaker's fundamental frequency is freer to deviate about a mean value. The present work presents a novel approach to discrimination between singing and speech that exploits the distribution of such deviations. The melody in singing is typically non known a priori, so the distribution cannot be measured directly. Instead, an approximation to its Fourier transform is proposed that allows the unknown melody to be treated as multiplicative noise. This feature vector is shown to be highly discriminative between speech and singing segments when coupled with a simple maximum likelihood classifier, outperforming prior work on real-world data.