One key finding in support of the hypothesis that written words are automatically parsed into component morphemes independently of the true morphological structure of the stimuli, so-called morpho-orthographic segmentation, is that suffixed nonword primes facilitate the visual recognition of a stem target (rapidifier-RAPIDE) whereas non-suffixed primes (rapiduit-RAPIDE) do not. However, Morris, Porter, Grainger, and Holcomb (Language & Cognitive Processes, 26(4–6), 558–599, 2011)reported equivalent priming from suffixed and non-suffixed nonword primes, hence questioning the morphological nature of prior findings. Here we provide a further investigation of masked priming with morphologically complex nonword primes with an aim to isolate factors that modulate the size of these priming effects. We conducted a masked primed lexical decision experiment in French, in which the same target (TRISTE) was preceded by a suffixed word (tristesse), a suffixed nonword (tristerie), a non-suffixed nonword (tristald), or an unrelated prime word (direction). Participants were split into two groups, based on their language proficiency. The results show that in the high proficiency group, comparable magnitudes of priming were obtained in all three related prime conditions (including the non-suffixed condition) relative to unrelated primes, whereas in the low proficiency group, priming was significantly reduced in the non-suffixed condition compared to the two suffixed conditions. These findings provide further evidence that individual differences in language proficiency can modulate the impact of morphological factors during reading, and an explanation for the discrepant findings in prior research.