diff --git a/tests/test_conditional_simulations.py b/tests/test_conditional_simulations.py
index 38d06b17d..94e740f68 100644
--- a/tests/test_conditional_simulations.py
+++ b/tests/test_conditional_simulations.py
@@ -217,6 +217,7 @@ def test_sweep_from_new_mutation_using_API(
     assert output.num_descendant_nodes == 1
     assert output.pop.generation > pop.generation
     if output.pop is not None:
+        assert output.mutation_index is not None
         assert output.pop.mcounts[output.mutation_index] == 2 * output.pop.N
         _ = output.pop.dump_tables_to_tskit()
         if output.pop.fixation_times[output.mutation_index] < output.pop.generation: