diff --git a/src/lib/ostream-buffer.c b/src/lib/ostream-buffer.c
index 84c7a2bb4a..2ebfbe82d3 100644
--- a/src/lib/ostream-buffer.c
+++ b/src/lib/ostream-buffer.c
@@ -53,6 +53,14 @@ o_stream_buffer_sendv(struct ostream_private *stream,
 	return ret;
 }
 
+static size_t
+o_stream_buffer_get_used_size(const struct ostream_private *stream)
+{
+	struct buffer_ostream *bstream = (struct buffer_ostream *)stream;
+
+	return bstream->buf->used;
+}
+
 struct ostream *o_stream_create_buffer(buffer_t *buf)
 {
 	struct buffer_ostream *bstream;
@@ -66,6 +74,7 @@ struct ostream *o_stream_create_buffer(buffer_t *buf)
 	bstream->ostream.seek = o_stream_buffer_seek;
 	bstream->ostream.sendv = o_stream_buffer_sendv;
 	bstream->ostream.write_at = o_stream_buffer_write_at;
+	bstream->ostream.get_used_size = o_stream_buffer_get_used_size;
 
 	bstream->buf = buf;
 	output = o_stream_create(&bstream->ostream, NULL, -1);