diff -r cd8b6f32ed79 -r 512ac874bb9d lib/scripts/ocamlc
--- a/lib/scripts/ocamlc	Tue Mar 19 16:14:59 2019 +0000
+++ b/lib/scripts/ocamlc	Wed Mar 20 17:33:06 2019 +0100
@@ -4,7 +4,7 @@
 #
 # Invoke ocamlc via "opam".
 
-if [ -d "$ISABELLE_OPAM_ROOT" ]
+if [ -e "$ISABELLE_OPAM_ROOT/config" ]
 then
   isabelle_opam config exec --switch "$ISABELLE_OCAML_VERSION" -- ocamlc "$@"
 else