diff --git a/nixos/lib/utils.nix b/nixos/lib/utils.nix
index 871fbb121d0..40d0854d968 100644
--- a/nixos/lib/utils.nix
+++ b/nixos/lib/utils.nix
@@ -12,6 +12,8 @@ rec {
   toShellPath = shell:
     if types.shellPackage.check shell then
       "/run/current-system/sw${shell.shellPath}"
+    else if types.package.check shell then
+      throw "${shell} is not a shell package"
     else
       shell;
 }