diff --git a/rtmp/rtmp.go b/rtmp/rtmp.go
index bf373377..9e6c202b 100644
--- a/rtmp/rtmp.go
+++ b/rtmp/rtmp.go
@@ -2112,7 +2112,7 @@ func C_RTMP_Write(r *C_RTMP, buf []byte) int {
 	// TODO: port RTMPPacket
 	var pkt = &r.m_write
 	var pend, enc []byte
-	size := len(data)
+	size := len(buf)
 	s2 := size
 	var ret, num int