diff --git a/src/output/plugins/AlsaOutputPlugin.cxx b/src/output/plugins/AlsaOutputPlugin.cxx
index b2316e865..cae0d04ac 100644
--- a/src/output/plugins/AlsaOutputPlugin.cxx
+++ b/src/output/plugins/AlsaOutputPlugin.cxx
@@ -312,6 +312,7 @@ private:
 
 	snd_pcm_sframes_t WriteFromPeriodBuffer() noexcept {
 		assert(!period_buffer.IsEmpty());
+		assert(period_buffer.IsFull());
 
 		auto frames_written = snd_pcm_writei(pcm, period_buffer.GetHead(),
 						     period_buffer.GetFrames(out_frame_size));