1 files changed, 1 insertions, 0 deletions
diff --git a/lib/compat.ml4 b/lib/compat.ml4
index aba2720456..312860a63e 100644
--- a/lib/compat.ml4
+++ b/lib/compat.ml4
@@ -80,6 +80,7 @@ module type LexerSig = sig
   module Error : sig
     type t
     exception E of t
+    val to_string : t -> string
   end
 end