diff --git a/librtmp/rtmp.c b/librtmp/rtmp.c
index 2bf9868..7266112 100644
--- a/librtmp/rtmp.c
+++ b/librtmp/rtmp.c
@@ -4210,7 +4210,7 @@ fail:
 static const AVal av_setDataFrame = AVC("@setDataFrame");
 
 int
-RTMP_Write(RTMP *r, char *buf, int size)
+RTMP_Write(RTMP *r, const char *buf, int size)
 {
   RTMPPacket *pkt = &r->m_write;
   char *pend, *enc;
diff --git a/librtmp/rtmp.h b/librtmp/rtmp.h
index c593cb7..14c9068 100644
--- a/librtmp/rtmp.h
+++ b/librtmp/rtmp.h
@@ -323,7 +323,7 @@ extern "C"
   bool RTMP_SendClientBW(RTMP *r);
   void RTMP_DropRequest(RTMP *r, int i, bool freeit);
   int RTMP_Read(RTMP *r, char *buf, int size);
-  int RTMP_Write(RTMP *r, char *buf, int size);
+  int RTMP_Write(RTMP *r, const char *buf, int size);
 
 /* hashswf.c */
   int RTMP_HashSWF(const char *url, unsigned int *size, unsigned char *hash,