diff --git a/libavformat/mmsh.c b/libavformat/mmsh.c
index d6e398200f..86a0575e59 100644
--- a/libavformat/mmsh.c
+++ b/libavformat/mmsh.c
@@ -394,7 +394,7 @@ static int64_t mmsh_seek(URLContext *h, int64_t pos, int whence)
     MMSContext *mms   = &mmsh->mms;
 
     if(pos == 0 && whence == SEEK_CUR)
-        return mms->asf_header_read_size + mms->remaining_in_len + mmsh->chunk_seq * mms->asf_packet_len;
+        return mms->asf_header_read_size + mms->remaining_in_len + mmsh->chunk_seq * (int64_t)mms->asf_packet_len;
     return AVERROR(ENOSYS);
 }