Using ocaml 3.10.1 and merlin 4.10.1 I get the following error when running the merlin-use command on emacs:

unknown flag "-package"

It looks like the ocamlmerlin command is being constructed with a -package option, as can be seen in this line

However, said flag is not referenced in the source code other than in the emacs file since 7a45a1f according to git-bisect

Works fine in merlin 3.3.6