diff --git a/lib/lists.nix b/lib/lists.nix
index 860bf5560797..8766d3f25d21 100644
--- a/lib/lists.nix
+++ b/lib/lists.nix
@@ -445,7 +445,7 @@ rec {
 
     :::
   */
-  flatten = x: if isList x then concatMap (y: flatten y) x else [ x ];
+  flatten = x: if isList x then concatMap flatten x else [ x ];
 
   /**
     Remove elements equal to `e` from a list.  Useful for `buildInputs`.