Are you on the Conservatives Enemies' list?

The other day I received an unidentified letter in the mail warning in big bold letters (exactly as written) “Are you on the Conservatives Enemies’ list?” I opened it eagerly to find that it was from the NDP telling me, among other things, of a data base operated by the Conservatives.  I was heartened to learn that I am likely not alone on that data base.  But I was also bothered by the bad grammar.  The envelope should have read: “Are you on the Conservatives’ Enemies list?”  Apparently the federal NDP, while no longer socialist, still has problems with its possessives.