diff --git a/AUTHORS.adoc b/AUTHORS.adoc
index b1ee4699..be70282a 100644
--- a/AUTHORS.adoc
+++ b/AUTHORS.adoc
@@ -13,3 +13,4 @@
 - Guillaume Bury (guigui)
 - JP Rodi
 - octachron (Florian Angeletti)
+- Johannes Kloos
diff --git a/src/data/CCPersistentHashtbl.ml b/src/data/CCPersistentHashtbl.ml
index 798b04f1..1fd06f67 100644
--- a/src/data/CCPersistentHashtbl.ml
+++ b/src/data/CCPersistentHashtbl.ml
@@ -89,8 +89,8 @@ module type S = sig
   (** Fresh copy of the table; the underlying structure is not shared
       anymore, so using both tables alternatively will be efficient *)
 
-  val merge : (key -> 'a option -> 'a option -> 'a option) ->
-              'a t -> 'a t -> 'a t
+  val merge : (key -> 'a option -> 'b option -> 'c option) ->
+              'a t -> 'b t -> 'c t
   (** Merge two tables together into a new table. The function's argument
       correspond to values associated with the key (if present); if the
       function returns [None] the key will not appear in the result. *)
@@ -576,7 +576,7 @@ module Make(H : HashedType) : S with type key = H.t = struct
         if mem t1 k then tbl
         else match f k None (Some v2) with
           | None -> tbl
-          | Some _ -> replace tbl k v2
+          | Some v' -> replace tbl k v'
       ) tbl t2
 
   (*$R
diff --git a/src/data/CCPersistentHashtbl.mli b/src/data/CCPersistentHashtbl.mli
index 908b9252..6fed4d96 100644
--- a/src/data/CCPersistentHashtbl.mli
+++ b/src/data/CCPersistentHashtbl.mli
@@ -96,8 +96,8 @@ module type S = sig
   (** Fresh copy of the table; the underlying structure is not shared
       anymore, so using both tables alternatively will be efficient *)
 
-  val merge : (key -> 'a option -> 'a option -> 'a option) ->
-              'a t -> 'a t -> 'a t
+  val merge : (key -> 'a option -> 'b option -> 'c option) ->
+              'a t -> 'b t -> 'c t
   (** Merge two tables together into a new table. The function's argument
       correspond to values associated with the key (if present); if the
       function returns [None] the key will not appear in the result. *)