diff --git a/libavformat/ffmdec.c b/libavformat/ffmdec.c
index 6bcb98c72e77b4243f1be2ea341d4e44da27ba5d..0c943fbe098b93e7edd1c6d8d47cfe5cdb59ebc6 100644
--- a/libavformat/ffmdec.c
+++ b/libavformat/ffmdec.c
@@ -285,7 +285,7 @@ static int ffm_read_header(AVFormatContext *s)
     /* get also filesize */
     if (pb->seekable) {
         ffm->file_size = avio_size(pb);
-        if (ffm->write_index)
+        if (ffm->write_index && 0)
             adjust_write_index(s);
     } else {
         ffm->file_size = (UINT64_C(1) << 63) - 1;