diff --git a/ucl/userbuiltin.go b/ucl/userbuiltin.go
index e38a2f8..7058b0e 100644
--- a/ucl/userbuiltin.go
+++ b/ucl/userbuiltin.go
@@ -165,7 +165,7 @@ func (ca CallArgs) bindArg(v interface{}, arg Object) error {
 		return bindProxyObject(v, t.v)
 	}
 
-	return nil
+	return bindProxyObject(v, reflect.ValueOf(arg))
 }
 
 func canBindArg(v interface{}, arg Object) bool {