diff --git a/libavcodec/h264.c b/libavcodec/h264.c
index 03d70b7d78..64ca5b7e66 100644
--- a/libavcodec/h264.c
+++ b/libavcodec/h264.c
@@ -3768,6 +3768,9 @@ static int decode_slice_header(H264Context *h, H264Context *h0){
                 s->avctx->time_base.den *= 2;
             av_reduce(&s->avctx->time_base.num, &s->avctx->time_base.den,
                       s->avctx->time_base.num, s->avctx->time_base.den, 1<<30);
+        }else if(!h->sps.time_scale && !s->avctx->frame_number){
+            s->avctx->time_base.den *=2;
+            h->sps.time_scale= s->avctx->time_base.den;
         }
     }