diff --git a/passes/cmds/bbox_derive.cc b/passes/cmds/bbox_derive.cc
index 758f793004a..a35e9629058 100644
--- a/passes/cmds/bbox_derive.cc
+++ b/passes/cmds/bbox_derive.cc
@@ -71,6 +71,8 @@ struct BboxDerivePass : Pass {
 
 				IdString derived_type = base->derive(d, cell->parameters);
 				Module *derived = d->module(derived_type);
+				log_assert(derived && "Failed to derive module\n");
+				log_debug("derived %s\n", derived_type.c_str());
 
 				if (!naming_attr.empty() && derived->has_attribute(naming_attr)) {
 					IdString new_name = RTLIL::escape_id(derived->get_string_attribute(naming_attr));