diff --git a/libavformat/rmenc.c b/libavformat/rmenc.c
index 17192ff275..a4e97a02e9 100644
--- a/libavformat/rmenc.c
+++ b/libavformat/rmenc.c
@@ -391,6 +391,11 @@ static int rm_write_video(AVFormatContext *s, const uint8_t *buf, int size, int
     /* Well, I spent some time finding the meaning of these bits. I am
        not sure I understood everything, but it works !! */
 #if 1
+    /* 0xFFFF is the maximal chunk size; header needs at most 7 + 4 + 12 B */
+    if (size > 0xFFFF - 7 - 4 - 12) {
+        av_log(s, AV_LOG_ERROR, "large packet size %d not supported\n", size);
+        return AVERROR_PATCHWELCOME;
+    }
     write_packet_header(s, stream, size + 7 + (size >= 0x4000)*4, key_frame);
     /* bit 7: '1' if final packet of a frame converted in several packets */
     avio_w8(pb, 0x81);