Without correctly identifying the molecular ion on a mass spectrum, the nitrogen rule generally cannot be applied to any arbitrary ion peak.
The chemical structure for N-[(1E)-1-phenylethylidene]methanamine and its EI mass spectrum are shown below. As expected, the odd value for the molecular ion at m/z 133 dictates an odd number of nitrogen atoms. However, the even value for the fragment ion peak at m/z 118 corresponds to the C8H8NO fragment with an odd number of nitrogen atoms. As such, the nitrogen rule is best reserved for the molecular ion peak as not to risk an incorrect assessment of the number of nitrogen atoms for an unknown compound.