diff --git a/builder.nix b/builder.nix
index d98ff37..1b139d6 100644
--- a/builder.nix
+++ b/builder.nix
@@ -19,7 +19,7 @@ let
       # scripts.
       funcPkgs = {
         __functor = self: arg: resolve arg;
-        src = getSrc root rcwd filter;
+        src = getSrc root rcwd filter id;
         inherit pkgs cwd rcwd d1 d2;
       };
 
diff --git a/lib.nix b/lib.nix
index 776e821..8a40eff 100644
--- a/lib.nix
+++ b/lib.nix
@@ -86,7 +86,7 @@ let
   #
   # Returns a filter that accepts zero to n paths relative to cwd. Captures the
   # whole source tree when called with zero arguments.
-  getSrc = root: rcwd: rejected:
+  getSrc = root: rcwd: rejected: outFunc:
     captureFunc (resolveSrc root rcwd) (srcs: let
 
       rejector = path: (all (x: path != x) rejected);
@@ -109,7 +109,7 @@ let
         (rejector path) &&
         ((srcFilter path) ||
          (srcDirFilter path)));
-    in (builtins.filterSource filter root));
+    in outFunc (builtins.filterSource filter root));
 
   self = {
     inherit doFileSuffix whichdo d1 d2 getSrc;