diff --git a/bytecomp/lambda.ml b/bytecomp/lambda.ml
index 68b03d280..3c1aaf26b 100644
--- a/bytecomp/lambda.ml
+++ b/bytecomp/lambda.ml
@@ -211,16 +211,23 @@ let const_unit = Const_pointer 0
 
 let lambda_unit = Lconst const_unit
 
+(* Build sharing keys *)
+(*
+   Those keys are later compared with Pervasives.compare.
+   For that reason, they should not include cycles.
+*)
+
 exception Not_simple
 
 let max_raw = 32
 
 let make_key e =
-  let count = ref 0
+  let count = ref 0   (* Used for controling size *)
   and make_key = Ident.make_key_generator () in
+  (* make_key is used for normalizing let-bound variables *)
   let rec tr_rec env e =
     incr count ;
-    if !count > max_raw then raise Not_simple ;
+    if !count > max_raw then raise Not_simple ; (* Too big ! *)
     match e with
     | Lvar id ->
       begin
@@ -264,11 +271,12 @@ let make_key e =
         Lassign (x,tr_rec env e)
     | Lsend (m,e1,e2,es,loc) ->
         Lsend (m,tr_rec env e1,tr_rec env e2,tr_recs env es,Location.none)
-    | Levent (e,evt) ->
-        Levent (tr_rec env e,evt)
     | Lifused (id,e) -> Lifused (id,tr_rec env e)
     | Lletrec _|Lfunction _
-    | Lfor _ | Lwhile _  ->
+    | Lfor _ | Lwhile _
+(* Beware: (PR#6412) the event argument to Levent
+   may include cyclic structure of type Type.typexpr *)
+    | Levent _  ->
         raise Not_simple
 
   and tr_recs env es = List.map (tr_rec env) es
diff --git a/bytecomp/matching.ml b/bytecomp/matching.ml
index 2e2eea9a2..88a348408 100644
--- a/bytecomp/matching.ml
+++ b/bytecomp/matching.ml
@@ -531,9 +531,7 @@ let up_ok_action act1 act2 =
   try
     let raw1 = tr_raw act1
     and raw2 = tr_raw act2 in
-    match raw1, raw2 with
-    | Lstaticraise (i1,[]), Lstaticraise (i2,[]) -> i1=i2
-    | _,_ -> raw1 = raw2
+    raw1 = raw2
   with
   | Exit -> false