diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
index 0c733a704bcd4ff344c3331a4b2191482b10399a..3f3721c58b04b444e617223c3e6e7f9ee61bb983 100644
--- a/libavformat/aviobuf.c
+++ b/libavformat/aviobuf.c
@@ -468,7 +468,7 @@ void put_tag(AVIOContext *s, const char *tag)
 
 static void fill_buffer(AVIOContext *s)
 {
-    uint8_t *dst= !s->max_packet_size && s->buf_end - s->buffer < s->buffer_size ? s->buf_ptr : s->buffer;
+    uint8_t *dst= !s->max_packet_size && s->buf_end - s->buffer < s->buffer_size ? s->buf_end : s->buffer;
     int len= s->buffer_size - (dst - s->buffer);
     int max_buffer_size = s->max_packet_size ? s->max_packet_size : IO_BUFFER_SIZE;