diff --git a/spot/graph/graph.hh b/spot/graph/graph.hh
index a11f5cc40..ba2ee41e2 100644
--- a/spot/graph/graph.hh
+++ b/spot/graph/graph.hh
@@ -1025,7 +1025,7 @@ namespace spot
     ///
     /// \param newst A vector indicating how each state should be renumbered.
     /// Use -1U to erase a state.
-    /// \param the number of states used (after renumbering)
+    /// \param used_states the number of states used (after renumbering)
     void defrag_states(std::vector<unsigned>&& newst, unsigned used_states)
     {
       assert(newst.size() == states_.size());