diff --git a/pkgs/lib/modules.nix b/pkgs/lib/modules.nix
index 1744d39d8de..9bd3b4aab8c 100644
--- a/pkgs/lib/modules.nix
+++ b/pkgs/lib/modules.nix
@@ -223,6 +223,7 @@ rec {
 
           value = decl // (with config; {
             inherit (config) isNotDefined;
+            isDefined = ! isNotDefined;
             declarations = declarationSources;
             definitions = definitionSources;
             config = strictResult;