Electronic structure in atoms and molecules modulates the amplitude and phase of high harmonic generation (HHG). We report measurements of the high harmonic spectral amplitude and phase in N2. The phase is measured interferometrically by beating the N2 harmonics with those of an Ar reference oscillator in a gas mixture. A rapid phase shift of 0.2 pi is observed in the vicinity of the HHG spectral minimum, where a shift of pi had been presumed [J. Itatani et al., Nature 432, 867 (2004)].