diff --git a/spot/twa/twagraph.cc b/spot/twa/twagraph.cc
index 2bcce5fed..0fc9aa292 100644
--- a/spot/twa/twagraph.cc
+++ b/spot/twa/twagraph.cc
@@ -368,11 +368,12 @@ namespace spot
     unsigned n_edges = e_vec.size();
     if (n_edges <= 1)
       {
-        if (n_states == 1)
+        if (n_states <= 1)
           return 0;
         // We don't have a very convenient way to resize the state
         // vector.
         std::vector<unsigned> remap(n_states, -1U);
+        SPOT_ASSUME(remap.data() != nullptr); // help GCC 9.4
         remap[0] = 0;
         get_graph().defrag_states(remap, 1);
         SPOT_ASSERT(num_states() == 1);
diff --git a/spot/twaalgos/split.cc b/spot/twaalgos/split.cc
index 8ac5682f3..11146837d 100644
--- a/spot/twaalgos/split.cc
+++ b/spot/twaalgos/split.cc
@@ -232,7 +232,11 @@ namespace spot
     // number of a BDD that as been (or will be) split, and begin/end
     // denotes a range of existing transition numbers that cover the
     // split.
-    typedef std::pair<unsigned, unsigned> cached_t;
+    //
+    // std::pair causes some noexcept warnings when used in
+    // robin_hood::unordered_map with GCC 9.4.  Use robin_hood::pair
+    // instead.
+    typedef robin_hood::pair<unsigned, unsigned> cached_t;
     robin_hood::unordered_map<unsigned, cached_t> split_cond;
 
     bdd all = aut->ap_vars();