diff --git a/pkgs/build-support/mkshell/default.nix b/pkgs/build-support/mkshell/default.nix
index 7ca4cc23c1d5..f85a11394cb5 100644
--- a/pkgs/build-support/mkshell/default.nix
+++ b/pkgs/build-support/mkshell/default.nix
@@ -14,8 +14,9 @@
 , ...
 }@attrs:
 let
-  mergeInputs = name: lib.concatLists (lib.catAttrs name
-    ([ attrs ] ++ inputsFrom));
+  mergeInputs = name:
+    (attrs.${name} or []) ++
+    (lib.subtractLists inputsFrom (lib.catAttrs name inputsFrom));
 
   rest = builtins.removeAttrs attrs [
     "packages"