diff --git a/libavformat/rmdec.c b/libavformat/rmdec.c
index 42f76491e2..c89fba4e7f 100644
--- a/libavformat/rmdec.c
+++ b/libavformat/rmdec.c
@@ -568,6 +568,8 @@ static int rm_read_header(AVFormatContext *s)
     }
 
     tag_size = avio_rb32(pb);
+    if (tag_size < 0)
+        return AVERROR_INVALIDDATA;
     avio_skip(pb, tag_size - 8);
 
     for(;;) {