diff --git a/libs/ardour/ardour/io.h b/libs/ardour/ardour/io.h
index 3f4fdc5797..d9fc412e27 100644
--- a/libs/ardour/ardour/io.h
+++ b/libs/ardour/ardour/io.h
@@ -104,6 +104,8 @@ public:
 
 	boost::shared_ptr<Bundle> bundle () { return _bundle; }
 
+	bool can_add_port (DataType) const;
+
 	int add_port (std::string connection, void *src, DataType type = DataType::NIL);
 	int remove_port (boost::shared_ptr<Port>, void *src);
 	int connect (boost::shared_ptr<Port> our_port, std::string other_port, void *src);
diff --git a/libs/ardour/io.cc b/libs/ardour/io.cc
index c6624b2e4a..52bbd59b5f 100644
--- a/libs/ardour/io.cc
+++ b/libs/ardour/io.cc
@@ -200,6 +200,19 @@ IO::connect (boost::shared_ptr<Port> our_port, string other_port, void* src)
 	return 0;
 }
 
+bool
+IO::can_add_port (DataType type) const
+{
+	switch (type) {
+		case DataType::NIL:
+			return false;
+		case DataType::AUDIO:
+			return true;
+		case DataType::MIDI:
+			return _ports.count ().n_midi() < 1;
+	}
+}
+
 int
 IO::remove_port (boost::shared_ptr<Port> port, void* src)
 {
@@ -269,6 +282,10 @@ IO::add_port (string destination, void* src, DataType type)
 		type = _default_type;
 	}
 
+	if (!can_add_port (type)) {
+		return -1;
+	}
+
 	ChanCount before = _ports.count ();
 	ChanCount after = before;
 	after.set (type, after.get (type) + 1);