Recently, in a series of papers, Joshua Spencer has introduced, defended, and developed a modified version of Neo-Russellianism (NR), namely Plenitudinous Russellianism (PR), according to which there are structurally identical but numerically distinct singular Russellian propositions (SRPs). PR claims to provide novel semantic solutions to all the major problems that NR faces with no radical revision in NR. In this paper, I introduce a semantic puzzle for PR: the view leads to the violation of the principle of substitutivity of co-referential proper names within simple (predicative) ‘that’-clauses (PS1). I consider different responses to my argument, and show that none of them is fully satisfactory for the Russellian. I conclude that PR needs to depart from NR more radically.