diff --git a/Machines/Atari/ST/AtariST.cpp b/Machines/Atari/ST/AtariST.cpp
index 187ee2af1..16583a7f5 100644
--- a/Machines/Atari/ST/AtariST.cpp
+++ b/Machines/Atari/ST/AtariST.cpp
@@ -469,6 +469,7 @@ class ConcreteMachine:
 				length -= cycles_until_video_event_;
 				video_ += cycles_until_video_event_;
 				cycles_until_video_event_ = video_->get_next_sequence_point();
+				assert(cycles_until_video_event_ > HalfCycles(0));
 
 				mfp_->set_timer_event_input(1, video_->display_enabled());
 				update_interrupt_input();
diff --git a/Outputs/CRT/CRT.cpp b/Outputs/CRT/CRT.cpp
index 84f30480d..a6e02aed9 100644
--- a/Outputs/CRT/CRT.cpp
+++ b/Outputs/CRT/CRT.cpp
@@ -294,6 +294,8 @@ void CRT::advance_cycles(int number_of_cycles, bool hsync_requested, bool vsync_
 // MARK: - stream feeding methods
 
 void CRT::output_scan(const Scan *const scan) {
+	assert(scan->number_of_cycles >= 0);
+
 	// Simplified colour burst logic: if it's within the back porch we'll take it.
 	if(scan->type == Scan::Type::ColourBurst) {
 		if(!colour_burst_amplitude_ && horizontal_flywheel_->get_current_time() < (horizontal_flywheel_->get_standard_period() * 12) >> 6) {