diff --git a/spot/twacube/twacube.cc b/spot/twacube/twacube.cc
index 26b4342d6..1ec95aa14 100644
--- a/spot/twacube/twacube.cc
+++ b/spot/twacube/twacube.cc
@@ -66,7 +66,7 @@ namespace spot
     return acc_;
   }
 
-  std::vector<std::string> twacube::get_ap()
+  std::vector<std::string> twacube::get_ap() const
   {
     return aps_;
   }
diff --git a/spot/twacube/twacube.hh b/spot/twacube/twacube.hh
index 1a8112a84..2a9a3885b 100644
--- a/spot/twacube/twacube.hh
+++ b/spot/twacube/twacube.hh
@@ -139,7 +139,7 @@ namespace spot
     acc_cond& acc();
 
     /// \brief Returns the names of the atomic properties.
-    std::vector<std::string> get_ap();
+    std::vector<std::string> get_ap() const;
 
     /// \brief This method creates a new state.
     unsigned new_state();