diff --git a/app/src/video_buffer.c b/app/src/video_buffer.c
index 660c6929..95cfd755 100644
--- a/app/src/video_buffer.c
+++ b/app/src/video_buffer.c
@@ -64,6 +64,7 @@ video_buffer_destroy(struct video_buffer *vb) {
 
 static void
 video_buffer_swap_frames(struct video_buffer *vb) {
+    sc_mutex_assert(&vb->mutex);
     AVFrame *tmp = vb->decoding_frame;
     vb->decoding_frame = vb->rendering_frame;
     vb->rendering_frame = tmp;
@@ -92,6 +93,7 @@ video_buffer_offer_decoded_frame(struct video_buffer *vb,
 
 const AVFrame *
 video_buffer_consume_rendered_frame(struct video_buffer *vb) {
+    sc_mutex_assert(&vb->mutex);
     assert(!vb->rendering_frame_consumed);
     vb->rendering_frame_consumed = true;
     fps_counter_add_rendered_frame(vb->fps_counter);