diff --git a/lib/spack/spack/build_systems/python.py b/lib/spack/spack/build_systems/python.py
index f075c43a17a..56d61a0191b 100644
--- a/lib/spack/spack/build_systems/python.py
+++ b/lib/spack/spack/build_systems/python.py
@@ -365,7 +365,7 @@ def urls(cls) -> Optional[List[str]]:
                 pypi = "/".join([name, file.replace("-", "_", name_dash_count)])
                 urls.append(f"https://files.pythonhosted.org/packages/source/{pypi[0]}/{pypi}")
             return urls
-        return None
+        return []
 
     @lang.classproperty
     def list_url(cls) -> Optional[str]:  # type: ignore[override]