diff --git a/tests/test_update_shape_pin_source.py b/tests/test_update_shape_pin_source.py
index 3b29edb9..8dbc229b 100644
--- a/tests/test_update_shape_pin_source.py
+++ b/tests/test_update_shape_pin_source.py
@@ -431,6 +431,16 @@ def opening(spec, *_pin_path):
         return real(str(upstream_and_project["upstream"]))
 
     monkeypatch.setattr(update_shape, "open_upstream", opening)
+    # THIS PROCESS READS `LANES_LANE`, THE CHILD READS A BLANK (#1299).
+    # `update_shape.main` prints its NEXT through `lane_trailer_argument`,
+    # which reads this process's environment. This module's `run` goes
+    # through `conftest.run_script`, which blanks the child's lane. In a
+    # shell that exports one, only the in-process NEXT ended
+    # `--trailer 'Lane: <lane>'`, and the two answers differed by that
+    # alone. Unset is no lane, as the blank is: the two read the same. With
+    # the guard in place the child gets no lane even without that blank,
+    # because `run_script` copies `os.environ` after the guard has run.
+    monkeypatch.delenv("LANES_LANE", raising=False)
     code = update_shape.main(["check", "--root", str(root)])
     out = capsys.readouterr().out
     passed = run(root, "check", *upstream_of(upstream_and_project))