diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index d4c9ff04cbd35981d073e8a999033b0bb70289c7..81757cd90a06db4017d4c3608bd47b9e30535e46 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -15,7 +15,7 @@ default:
 
 variables:
   DEFAULT: "master"
-  OCAML: "4.11"
+  OCAML: "4.13"
 
 ################################################################################
 ### TESTS
diff --git a/meta_simplify.ml b/meta_simplify.ml
index ca7e9d340d2e0114e61a5e69fd6e1c873099e264..1a9bf036557d2fcbdcb61ec8342e402964d39aca 100644
--- a/meta_simplify.ml
+++ b/meta_simplify.ml
@@ -28,7 +28,7 @@ open Cil_types
 *)
 let is_not_orig_variable lv =
   lv.lv_name = "__retres" ||
-  Extlib.string_prefix "__fc_" lv.lv_name ||
+  String.starts_with ~prefix:"__fc_" lv.lv_name ||
   match lv.lv_origin with
     | Some v ->
       not v.vsource || v.vtemp ||