diff --git a/libavcodec/flashsv.c b/libavcodec/flashsv.c
index 2ba32641b7..ac57f9f68a 100644
--- a/libavcodec/flashsv.c
+++ b/libavcodec/flashsv.c
@@ -377,6 +377,11 @@ static int flashsv_decode_frame(AVCodecContext *avctx, void *data,
                 }
 
                 if (has_diff) {
+                    if (!s->keyframe) {
+                        av_log(avctx, AV_LOG_ERROR,
+                               "inter frame without keyframe\n");
+                        return AVERROR_INVALIDDATA;
+                    }
                     s->diff_start  = get_bits(&gb, 8);
                     s->diff_height = get_bits(&gb, 8);
                     av_log(avctx, AV_LOG_DEBUG,