diff --git a/README.txt b/README.txt
index 46ac724de67805d8391968acf5e8c8420d6f785f..34bfca46c780fd249e4b3e45f599230882572994 100644
--- a/README.txt
+++ b/README.txt
@@ -27,7 +27,7 @@ Reference article:
   DOI: https://doi.org/10.1016/j.tcs.2018.02.003
 
 Requirements:
-  OCaml (requires 4.03.0, tested with version 4.09.0)
+  OCaml (requires 4.03.0, tested with versions 4.09.0 and 4.13.1)
   Clingo 5 (tested with version 5.4.0)
 
 
diff --git a/main.ml b/main.ml
index b43fb7b77a5c90af9ac99a644ed54970621ee238..4a9f89efc595dea0fc3ea815519adb4109f51ae9 100644
--- a/main.ml
+++ b/main.ml
@@ -159,7 +159,7 @@ type varpredec = mult list ;;
 
 (** Comparison functions **)
 let varequals = (=) ;;
-let multcompare = compare ;;
+let multcompare = String.compare ;;