diff --git a/passes/hierarchy/submod.cc b/passes/hierarchy/submod.cc
index 3494837..7b5e110 100644
--- a/passes/hierarchy/submod.cc
+++ b/passes/hierarchy/submod.cc
@@ -141,7 +141,7 @@
 			else {
 				auto sig = sigmap(wire);
 				for (auto c : sig.chunks())
-					if (c.wire->port_output) {
+					if (c.wire && c.wire->port_output) {
 						flags.is_ext_used = true;
 						break;
 					}