This study attempts to explain a systematic phenomenon that has been described in interlanguage grammars crosslinguistically: Null-Prep, which consists of omitting the obligatory preposition in certain movement constructions. We propose that Null-Prep is not related to lack of knowledge of "wh"-movement, as previously assumed, but to structural complexity; indeed, we consider Null-Prep a movement-derived structure. With evidence from prepositional relative clauses, "wh"-interrogatives, and sluicing constructions in first language (L1) and second language (L2) Spanish (English and Arabic L1s), we predict the potential appearance of the Null-Prep with a two-way complexity hierarchy that takes into account the syntactic position displaced, as well as its derivational complexity, in such a way that we calculate Null-Prep to occur more often in Relative Clauses, followed by Sluicing, and finally by Questions. This scalar phenomenon uniformly applies to all participants, native and L2 learners, emphasizing its universal nature.