diff --git a/backends/aiger/xaiger.cc b/backends/aiger/xaiger.cc
index f7c757754..875a2ec03 100644
--- a/backends/aiger/xaiger.cc
+++ b/backends/aiger/xaiger.cc
@@ -231,7 +231,7 @@ struct XAigerWriter
 								co_bits.emplace_back(b, 0);
 							else {
 								output_bits.insert(b);
-								if (!b.wire->port_input)
+								if (b.wire && !b.wire->port_input)
 									unused_bits.erase(b);
 							}
 						}