diff --git a/src/hq_interop.c b/src/hq_interop.c
index 9d0d56635..9af937f70 100644
--- a/src/hq_interop.c
+++ b/src/hq_interop.c
@@ -98,8 +98,8 @@ static size_t hq_interop_snd_buf(struct conn_stream *cs, struct buffer *buf,
 			if (fsize > count)
 				fsize = count;
 
-			if (b_size(&outbuf) < fsize)
-				fsize = b_size(&outbuf);
+			if (b_room(&outbuf) < fsize)
+				fsize = b_room(&outbuf);
 
 			if (!fsize) {
 				qcs->flags |= QC_SF_BLK_MROOM;