diff --git a/pyproject.toml b/pyproject.toml
index 422f75739..3e5701d78 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -5,7 +5,7 @@ test-command = [
   "python {package}/testdata/pymod/import_rdb.py",
   "python {package}/testdata/pymod/import_tl.py",
   "python {package}/testdata/pymod/import_lib.py",
-  "python {package}/testdata/pymod/import_lay.py",
+  "TESTSRC={package} python {package}/testdata/pymod/import_lay.py",
   "python {package}/testdata/pymod/pya_tests.py"
 ]
 # Disable building PyPy wheels on all platforms