diff --git a/units/tests/api.rs b/units/tests/api.rs
index 373780833..b4dc3aa6e 100644
--- a/units/tests/api.rs
+++ b/units/tests/api.rs
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0
+
 //! Test the API surface of `units`.
 //!
 //! The point of these tests are to check the API surface as opposed to test the API functionality.