diff --git a/pkgs/build-support/mkshell/default.nix b/pkgs/build-support/mkshell/default.nix
index f85a11394cb57..27ee7e3226241 100644
--- a/pkgs/build-support/mkshell/default.nix
+++ b/pkgs/build-support/mkshell/default.nix
@@ -16,7 +16,7 @@
 let
   mergeInputs = name:
     (attrs.${name} or []) ++
-    (lib.subtractLists inputsFrom (lib.catAttrs name inputsFrom));
+    (lib.subtractLists inputsFrom (lib.flatten (lib.catAttrs name inputsFrom)));
 
   rest = builtins.removeAttrs attrs [
     "packages"