diff --git a/macosx/coremidi/JackCoreMidiOutputPort.cpp b/macosx/coremidi/JackCoreMidiOutputPort.cpp
index fe1f2d6f..72eea5d4 100644
--- a/macosx/coremidi/JackCoreMidiOutputPort.cpp
+++ b/macosx/coremidi/JackCoreMidiOutputPort.cpp
@@ -55,6 +55,7 @@ JackCoreMidiOutputPort::Execute()
     for (;;) {
         MIDIPacket *packet = MIDIPacketListInit(packet_list);
         assert(packet);
+        assert(thread_queue);
         if (! event) {
             event = thread_queue->DequeueEvent((long) 0);
         }