diff --git a/nix/readtree/BUILD b/nix/readtree/BUILD
new file mode 100644
index 0000000..4a80df2
--- /dev/null
+++ b/nix/readtree/BUILD
@@ -0,0 +1,2 @@
+# Used by //go/workspace tests.
+exports_files(["default.nix"])