diff --git a/gtk2_ardour/audio_clock.cc b/gtk2_ardour/audio_clock.cc
index 9fb3901d88..dcbdb11c85 100644
--- a/gtk2_ardour/audio_clock.cc
+++ b/gtk2_ardour/audio_clock.cc
@@ -101,6 +101,7 @@ AudioClock::AudioClock (const string& clock_name, bool transient, const string&
 	, corner_radius (4)
 	, font_size (10240)
 	, editing (false)
+	, bbt_reference_time (timepos_t::from_superclock (-1)) /* needs to be some negative value but SR may not be available at init */
 	, last_pdelta (0)
 	, last_sdelta (0)
 	, dragging (false)