diff --git a/libavcodec/shorten.c b/libavcodec/shorten.c
index 46d3b7a615..b846aa2486 100644
--- a/libavcodec/shorten.c
+++ b/libavcodec/shorten.c
@@ -584,7 +584,7 @@ static int shorten_decode_frame(AVCodecContext *avctx, AVFrame *frame,
         if (avpkt->size) {
             int max_framesize = s->blocksize * s->channels * 8;
             void *tmp_ptr;
-
+            unsigned old_allocated_bitstream_size = s->allocated_bitstream_size;
             tmp_ptr = av_fast_realloc(s->bitstream, &s->allocated_bitstream_size,
                                       max_framesize + AV_INPUT_BUFFER_PADDING_SIZE);
             if (!tmp_ptr) {
@@ -592,9 +592,8 @@ static int shorten_decode_frame(AVCodecContext *avctx, AVFrame *frame,
                 return AVERROR(ENOMEM);
             }
             s->bitstream = tmp_ptr;
-            if (max_framesize > s->max_framesize)
-                memset(s->bitstream + s->max_framesize, 0, (max_framesize - s->max_framesize) +
-                                                            AV_INPUT_BUFFER_PADDING_SIZE);
+            if (s->allocated_bitstream_size > old_allocated_bitstream_size)
+                memset(s->bitstream + old_allocated_bitstream_size, 0, s->allocated_bitstream_size - old_allocated_bitstream_size);
             s->max_framesize = FFMAX(s->max_framesize, max_framesize);
             *got_frame_ptr = 0;
             goto finish_frame;