diff --git a/libs/ardour/ltc_slave.cc b/libs/ardour/ltc_slave.cc
index 6a2f71dacb..8b3a4eb966 100644
--- a/libs/ardour/ltc_slave.cc
+++ b/libs/ardour/ltc_slave.cc
@@ -525,6 +525,11 @@ LTC_TransportMaster::process_ltc(samplepos_t const now)
 void
 LTC_TransportMaster::pre_process (ARDOUR::pframes_t nframes, samplepos_t now, boost::optional<samplepos_t> session_pos)
 {
+	if (!_port) {
+		reset (true);
+		return;
+	}
+
 	Sample* in = (Sample*) AudioEngine::instance()->port_engine().get_buffer (_port->port_handle(), nframes);
 	sampleoffset_t skip;
 
diff --git a/libs/ardour/midi_clock_slave.cc b/libs/ardour/midi_clock_slave.cc
index c650b993ed..b76182f1fb 100644
--- a/libs/ardour/midi_clock_slave.cc
+++ b/libs/ardour/midi_clock_slave.cc
@@ -119,6 +119,14 @@ void
 MIDIClock_TransportMaster::pre_process (MIDI::pframes_t nframes, samplepos_t now, boost::optional<samplepos_t> session_pos)
 {
 	/* Read and parse incoming MIDI */
+	if (!_midi_port) {
+		_bpm = 0.0;
+		_running = false;
+		_current_delta = 0;
+		midi_clock_count = 0;
+		DEBUG_TRACE (DEBUG::MidiClock, "No MIDI Clock port registered");
+		return;
+	}
 
 	DEBUG_TRACE (DEBUG::MidiClock, string_compose ("preprocess with lt = %1 @ %2, running ? %3\n", current.timestamp, now, _running));
 
diff --git a/libs/ardour/mtc_slave.cc b/libs/ardour/mtc_slave.cc
index 5a620c74b0..22dda80cb1 100644
--- a/libs/ardour/mtc_slave.cc
+++ b/libs/ardour/mtc_slave.cc
@@ -131,6 +131,12 @@ MTC_TransportMaster::pre_process (MIDI::pframes_t nframes, samplepos_t now, boos
 
 	maybe_reset ();
 
+	if (!_midi_port) {
+		_current_delta = 0;
+		DEBUG_TRACE (DEBUG::MTC, "No MTC port registered");
+		return;
+	}
+
 	_midi_port->read_and_parse_entire_midi_buffer_with_no_speed_adjustment (nframes, parser, now);
 
 	if (session_pos) {