diff --git a/libavformat/rtpdec_rfc4175.c b/libavformat/rtpdec_rfc4175.c
index e9c62c1389..490db87520 100644
--- a/libavformat/rtpdec_rfc4175.c
+++ b/libavformat/rtpdec_rfc4175.c
@@ -205,8 +205,11 @@ static int rfc4175_handle_packet(AVFormatContext *ctx, PayloadContext *data,
         if (length > payload_len)
             length = payload_len;
 
+        if (line < 1)
+            return AVERROR_INVALIDDATA;
+
         /* prevent ill-formed packets to write after buffer's end */
-        copy_offset = (line * data->width + offset) * data->pgroup / data->xinc;
+        copy_offset = ((line - 1) * data->width + offset) * data->pgroup / data->xinc;
         if (copy_offset + length > data->frame_size)
             return AVERROR_INVALIDDATA;