diff --git a/src/t8_forest/t8_forest_iterate.cxx b/src/t8_forest/t8_forest_iterate.cxx
index 609e229c23..b870fb4b9e 100644
--- a/src/t8_forest/t8_forest_iterate.cxx
+++ b/src/t8_forest/t8_forest_iterate.cxx
@@ -357,6 +357,7 @@ t8_forest_iterate_replace (t8_forest_t forest_new, t8_forest_t forest_old, t8_fo
     t8_eclass_t eclass = t8_forest_get_tree_class (forest_new, itree);
     T8_ASSERT (eclass == t8_forest_get_tree_class (forest_old, itree));
     t8_eclass_scheme_c *ts = t8_forest_get_eclass_scheme (forest_new, eclass);
+    // Check that the two forests use the same scheme.
     T8_ASSERT (ts == t8_forest_get_eclass_scheme (forest_old, eclass));
 
     t8_locidx_t ielem_new = 0;