diff --git a/src/PatriciaTree.mli b/src/PatriciaTree.mli
index 3d24031..e8fccb0 100644
--- a/src/PatriciaTree.mli
+++ b/src/PatriciaTree.mli
@@ -19,6 +19,8 @@
 (*  for more details (enclosed in the file LICENSE).                      *)
 (**************************************************************************)
 
+exception Not_found
+
 (** Association maps from key to values, and sets, implemented with
     Patricia Trees, allowing fast merge operations by making use of
     physical equality between subtrees; and custom implementation of