diff --git a/libavcodec/dnxhdenc.c b/libavcodec/dnxhdenc.c
index 59e3f73689..6e9b54de4f 100644
--- a/libavcodec/dnxhdenc.c
+++ b/libavcodec/dnxhdenc.c
@@ -478,6 +478,7 @@ static av_cold int dnxhd_encode_init(AVCodecContext *avctx)
     if (ctx->cid_table->frame_size == DNXHD_VARIABLE) {
         ctx->frame_size = ff_dnxhd_get_hr_frame_size(ctx->cid,
                                                      avctx->width, avctx->height);
+        av_assert0(ctx->frame_size >= 0);
         ctx->coding_unit_size = ctx->frame_size;
     } else {
         ctx->frame_size = ctx->cid_table->frame_size;