diff --git a/pkgs/build-support/buildenv/default.nix b/pkgs/build-support/buildenv/default.nix
index e694a9b838e1..0f95f118d4bf 100644
--- a/pkgs/build-support/buildenv/default.nix
+++ b/pkgs/build-support/buildenv/default.nix
@@ -109,7 +109,7 @@ lib.makeOverridable (
           ;
         pathsToLinkJSON = builtins.toJSON pathsToLink;
         pkgs = builtins.toJSON chosenOutputs;
-        extraPathsFrom = lib.optional includeClosures (writeClosure pathsForClosure);
+        extraPathsFrom = lib.optionalString includeClosures (writeClosure pathsForClosure);
         preferLocalBuild = true;
         allowSubstitutes = false;
         # XXX: The size is somewhat arbitrary