diff --git a/pkgs/build-support/trivial-builders/default.nix b/pkgs/build-support/trivial-builders/default.nix
index d3cb22a1f535..2af36008c7a5 100644
--- a/pkgs/build-support/trivial-builders/default.nix
+++ b/pkgs/build-support/trivial-builders/default.nix
@@ -87,6 +87,10 @@ rec {
       inherit buildCommand name;
       passAsFile = [ "buildCommand" ]
         ++ (derivationArgs.passAsFile or []);
+      pos = let args = builtins.attrNames derivationArgs; in
+        if builtins.length args > 0
+        then builtins.unsafeGetAttrPos (builtins.head args) derivationArgs
+        else null;
     }
     // (lib.optionalAttrs runLocal {
           preferLocalBuild = true;