diff --git a/tests/test_conditional_simulations.py b/tests/test_conditional_simulations.py
index 6965372fe..76814af90 100644
--- a/tests/test_conditional_simulations.py
+++ b/tests/test_conditional_simulations.py
@@ -244,8 +244,9 @@ def test_sweep_from_new_mutation_using_API(
                             )
         else:
             assert prune_selected is True
-            # TODO: how can this not be true?
-            # assert len(pop.fixations) > 0, f"{prune_selected}"
+            assert len(output.pop.fixations) > 0
+            assert output.fixation_index is not None
+            assert output.fixation_index < len(output.pop.fixations)
 
         ti = fwdpy11.TreeIterator(output.pop.tables, output.pop.alive_nodes)
         for t in ti: