Jbuilder: confused about including C code

I am trying to create a library including both OCaml and C code. I use the c_names field to specify C files to include, like so:

echo "This C file does not compile" > exists.c
echo "let x = 42" > a.ml
cat << EOF > jbuild
(jbuild_version 1)
 ((name            mylib)
  (modules         (a))
  (c_names         (exists doesnotexist))
jbuilder build mylib.cmxa --verbose

This does not attempt to even compile my C files (it does not either complain that one of them does not exist). I am puzzled because this looks the same as what is done for instance in async ssl. (Also, if you look at the --verbose output the final command has -cclib -lmylib_stubs: this native library does not exist AFAICS but the command still succeeds!)

Submitted an issue: https://github.com/janestreet/jbuilder/issues/131