diff --git a/libs/ardour/session.cc b/libs/ardour/session.cc
index bf169dea7f..375e70d0c6 100644
--- a/libs/ardour/session.cc
+++ b/libs/ardour/session.cc
@@ -7896,7 +7896,7 @@ Session::build_available_physical_audio_io_lists (vector<string>& physinputs, ve
 	assert (physoutputs.size() <= output_channel_mask.size());
 
 	vector<string>::size_type n = 0;
-	for (vector<string>::iterator i = physinputs.begin(); i != physinputs.end(); ++i) {
+	for (vector<string>::iterator i = physinputs.begin(); i != physinputs.end(); ++n) {
 		if (!input_channel_mask[n]) {
 			i = physinputs.erase (i);
 		} else {
@@ -7904,7 +7904,7 @@ Session::build_available_physical_audio_io_lists (vector<string>& physinputs, ve
 		}
 	}
 	n = 0;
-	for (vector<string>::iterator o = physoutputs.begin(); o != physoutputs.end(); ++o) {
+	for (vector<string>::iterator o = physoutputs.begin(); o != physoutputs.end(); ++n) {
 		if (!output_channel_mask[n]) {
 			o = physoutputs.erase (o);
 		} else {