diff --git a/spot/twaalgos/aiger.cc b/spot/twaalgos/aiger.cc
index c5a2f8961..659dd281d 100644
--- a/spot/twaalgos/aiger.cc
+++ b/spot/twaalgos/aiger.cc
@@ -1331,6 +1331,7 @@ namespace spot
 
     std::deque<unsigned> todo;
     todo.push_back(0);
+    assert(n_max_states > 0);
     std::vector<bool> seen(n_max_states, false);
     seen[0] = true;