OCaml and Opam: unbound module Core

I'm trying to get an OCaml environment set up, and I've followed the instructions from appendix A of the Real World OCaml beta. I set up opam, and installed a version of OCaml with the command

$ opam switch 4.01.0dev+trunk

which passed fine. I then did an

$ eval `opam config env`

to pull in the changes. I'm running the correct top level, as

$ which ocaml

outputs

/home/bryan/.opam/4.01.0dev+trunk/bin/ocaml

I installed the Core package from Jane street, with the command

$ opam install core

Both ocamlfind and opam search show that the package was installed correctly. However when I try to open it either from the repl or in a file, I get the error 'unbound module Core'. e.g.

$ ocaml# open Core;;Error: Unbound module Core

Is there something I'm missing here? Why can't OCaml find my installed module?