Since Chomsky(1977) first claimed, it has been widely assumed that relative clauses contain wh-movement. The parallelism that exists between wh-interrogatives and relative clauses regarding `preposition stranding` and `pied-piping` seems to support this claim. However, there seems to be an exception to this parallelism. In wh-interrogatives the accusative particle `a` is required to appear, whereas in relative clauses it can be omitted. Thus, the aim of this paper is to give a generative account of why Spanish allows the accusative particle `a` to be omitted in relative clauses. For this, we first assume that the accusative particle `a` is an affix which needs to be attached to an adjacent lexical host. By extending the `Repair by Ellipsis Strategy` proposed by Lasnik(2002), which is a kind of last-resort we argue that the omission of the particle is to repair the violation of the requirement that an affix must attach to an appropriate host. In addition, we also try to account for why the definite article is required to appear when the accusative particle `a` is present. We claim that the definite article is a dummy element and that its insertion is another kind of last resort strategy which `rescues` an ill-formed structure.