diff --git a/libs/midi++2/port.cc b/libs/midi++2/port.cc
index fa052ef33c..600886a94d 100644
--- a/libs/midi++2/port.cc
+++ b/libs/midi++2/port.cc
@@ -173,6 +173,8 @@ Port::clock (timestamp_t timestamp)
 void
 Port::cycle_start (nframes_t nframes)
 {
+	assert (_jack_port);
+	
 	_currently_in_cycle = true;
 	_nframes_this_cycle = nframes;