diff --git a/kwant/tests/test_builder.py b/kwant/tests/test_builder.py
index 9bd369d66a3154ea1406abd013e6c7fb2110127d..9c83ac594e698fac82f36bdb63e1197acb25035b 100644
--- a/kwant/tests/test_builder.py
+++ b/kwant/tests/test_builder.py
@@ -715,6 +715,7 @@ def test_fill():
     target = builder.Builder()
     added_sites = target.fill(template_1d, line_200, g(0, 0))
     assert len(added_sites) == 200
+    # raise warning if target already contains all starting sites
     with warns(RuntimeWarning):
         target.fill(template_1d, line_200, g(0, 0))