diff --git a/backends/aiger/aiger.cc b/backends/aiger/aiger.cc
index 5b07a7195..e8b8e32ce 100644
--- a/backends/aiger/aiger.cc
+++ b/backends/aiger/aiger.cc
@@ -713,7 +713,7 @@ struct AigerWriter
 				}
 
 				if (wire->port_output) {
-					int o = ordered_outputs.at(sig[i]);
+					int o = ordered_outputs.at(SigSpec(wire, i));
 					output_lines[o] += stringf("output %d %d %s\n", o, index, log_id(wire));
 				}
 
diff --git a/tests/aiger/io.ys b/tests/aiger/io.ys
new file mode 100644
index 000000000..eae7f358d
--- /dev/null
+++ b/tests/aiger/io.ys
@@ -0,0 +1,10 @@
+read_verilog <<EOF
+module bad(
+	input in,
+	output reg [1:0] out
+);
+	assign out = {in, 1'b0};
+endmodule
+EOF
+proc
+write_aiger -vmap /dev/null /dev/null