[Haskell-beginners] Stack and packages not in the snapshot

Good morning list, I have started to use stack with Stackage LTS for building my projects because I like the idea of a stable snapshot of packages known to work together. Then I tried to build "x509-util-1.5.2". This package fails to build and this is already known by upstream. And lo, "x509-util" is not in Stackage LTS 3.2, which makes sense. But what surprised me is that stack attempted to build a package not in the snapshot, when my resolver is "lts-3.2" (and no extra-deps). Is there a way to configure stack to build packages from the snapshot only? Or does it already work this way, only trying "x509-util" because *I* said "stack build x509-util"? Thanks in advance. -- Thomas Koster