diff --git a/lib/spack/spack/config.py b/lib/spack/spack/config.py
index 3565f17a771..6b5b3dfd623 100644
--- a/lib/spack/spack/config.py
+++ b/lib/spack/spack/config.py
@@ -255,7 +255,7 @@
             },
             'dependency_selection': {
                 'type': 'string',
-                'enum': ['None', 'Direct', 'All']
+                'enum': ['none', 'direct', 'all']
             },
             'module_file_configuration': {
                 'type': 'object',