diff --git a/src/data/CCGraph.ml b/src/data/CCGraph.ml
index 0217f151..250b7189 100644
--- a/src/data/CCGraph.ml
+++ b/src/data/CCGraph.ml
@@ -414,7 +414,7 @@ module SCC = struct
     mutable min_id: int; (* min ID of the vertex' scc *)
     id: int;  (* ID of the vertex *)
     mutable on_stack: bool;
-    mutable vertex: 'v;
+    vertex: 'v;
   }
 
   let mk_cell v n = {