diff --git a/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.ml b/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.ml
index a114fc9fb5f56edc2a434bfb9783996ccc8122c7..27c45d86c695b99cd7435ef06100b6576e343fbe 100644
--- a/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.ml
+++ b/src/proto_alpha/lib_protocol/sc_rollup_proof_repr.ml
@@ -153,7 +153,7 @@ let produce pvm_and_state inbox_context inbox_history inbox commit_level =
             inbox_context
             inbox_history
             inbox
-            (l, n)
+            (l, Z.succ n)
         in
         return (Some p, i)
   in