diff --git a/libavcodec/h264_slice.c b/libavcodec/h264_slice.c
index 8c3b9f354c..05a6ccfa07 100644
--- a/libavcodec/h264_slice.c
+++ b/libavcodec/h264_slice.c
@@ -1459,6 +1459,8 @@ int ff_h264_decode_slice_header(H264Context *h, H264Context *h0)
             return AVERROR_INVALIDDATA;
         }
 
+        av_assert1(first_slice);
+
         ff_h264_flush_change(h);
 
         if ((ret = get_pixel_format(h, 1)) < 0)