diff --git a/CHANGES.md b/CHANGES.md
index bc48c8ac7..b6cb7ef52 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -3,6 +3,7 @@
 ## Fixes
 
 - Fix hover on method calls not showing the type. (#1553, fixes #1552)
+- Fix error on opening `.mll` files (#1557)
 
 # 1.23.0
 
diff --git a/ocaml-lsp-server/src/document.ml b/ocaml-lsp-server/src/document.ml
index ec1ec2956..fdee76287 100644
--- a/ocaml-lsp-server/src/document.ml
+++ b/ocaml-lsp-server/src/document.ml
@@ -8,7 +8,7 @@ module Kind = struct
 
   let of_fname_opt p =
     match Filename.extension p with
-    | ".ml" | ".eliom" | ".re" -> Some Impl
+    | ".ml" | ".eliom" | ".re" | ".mll" | ".mly" -> Some Impl
     | ".mli" | ".eliomi" | ".rei" -> Some Intf
     | _ -> None
   ;;