diff --git a/spot/twaalgos/degen.cc b/spot/twaalgos/degen.cc
index a2fa0ce26..11092ddac 100644
--- a/spot/twaalgos/degen.cc
+++ b/spot/twaalgos/degen.cc
@@ -1100,7 +1100,7 @@ namespace spot
 
   std::vector<acc_cond::mark_t>
   propagate_marks_vector(const const_twa_graph_ptr& aut,
-                         scc_info* si)
+                         const scc_info* si)
   {
     bool own_si = true;
     if (si)
@@ -1160,7 +1160,7 @@ namespace spot
     return marks;
   }
 
-  void propagate_marks_here(twa_graph_ptr& aut, scc_info* si)
+  void propagate_marks_here(twa_graph_ptr& aut, const scc_info* si)
   {
     auto marks = propagate_marks_vector(aut, si);
     for (auto& e: aut->edges())
diff --git a/spot/twaalgos/degen.hh b/spot/twaalgos/degen.hh
index 751c93336..281ba2ef5 100644
--- a/spot/twaalgos/degen.hh
+++ b/spot/twaalgos/degen.hh
@@ -172,10 +172,10 @@ namespace spot
   /// @{
   SPOT_API std::vector<acc_cond::mark_t>
   propagate_marks_vector(const const_twa_graph_ptr& aut,
-                         scc_info* si = nullptr);
+                         const scc_info* si = nullptr);
 
   SPOT_API void
   propagate_marks_here(twa_graph_ptr& aut,
-                       scc_info* si = nullptr);
+                       const scc_info* si = nullptr);
   /// @}
 }