diff --git a/include/mata/nfa/nfa.hh b/include/mata/nfa/nfa.hh
index 365035d56..48faa4427 100644
--- a/include/mata/nfa/nfa.hh
+++ b/include/mata/nfa/nfa.hh
@@ -322,6 +322,7 @@ public:
 
     /**
      * @brief Is the automaton flat?
+     * 
      * Flat automaton is an NFA whose every SCC is a simple loop. Basically each state in an 
      * SCC has at most one successor within this SCC.
      * 
diff --git a/include/mata/utils/ord-vector.hh b/include/mata/utils/ord-vector.hh
index d2a2e0706..909fa4da3 100644
--- a/include/mata/utils/ord-vector.hh
+++ b/include/mata/utils/ord-vector.hh
@@ -267,7 +267,7 @@ public:
     /**
      * Check whether @p key exists in the ordered vector.
      */
-    bool contains(const Key& key) { return find(key) != end(); }
+    bool contains(const Key& key) const { return find(key) != end(); }
 
     /**
      * @brief Remove @p k from sorted vector.