diff --git a/dbus/dbus-marshal-recursive.c b/dbus/dbus-marshal-recursive.c
index cb10854..512beed 100644
--- a/dbus/dbus-marshal-recursive.c
+++ b/dbus/dbus-marshal-recursive.c
@@ -1845,6 +1845,7 @@ writer_recurse_array (DBusTypeWriter   *writer,
       /* sub is a toplevel/outermost array so we need to write the type data */
 
       /* alloc space for array typecode, element signature */
+      _dbus_assert (writer->type_str);
       if (!_dbus_string_alloc_space (writer->type_str, 1 + contained_type_len))
         return FALSE;