diff --git a/tool/mk_builtin_loader.rb b/tool/mk_builtin_loader.rb
index f28f48f570024ecabfe8f37716b9547152e161c1..505a256ba8aba928a058aac6c017312158a63548 100644
--- a/tool/mk_builtin_loader.rb
+++ b/tool/mk_builtin_loader.rb
@@ -79,6 +79,7 @@ def collect_builtin base, tree, name, bs, inlines, params = nil
       tree = tree[3]
       next
     when :defs
+      params = collect_params(tree[4])
       tree = tree[5]
       next
     when :class