diff --git a/src/h3.c b/src/h3.c
index b4278efda..aca564fbf 100644
--- a/src/h3.c
+++ b/src/h3.c
@@ -2213,13 +2213,20 @@ static int h3_resp_headers_send(struct qcs *qcs, struct htx *htx)
 		goto end;
 	}
 
-	/* Buffer allocated just now : must be enough for frame type + length as a max varint size */
-	BUG_ON(b_room(res) < 5);
+	/* Reserve space for frame type + length as a max varint size. */
+	if (unlikely(b_contig_space(res) < 5)) {
+		/* Most of the times, h3_resp_headers_send() is only called one
+		 * time per stream, so buffer will be empty. However, this
+		 * assumption is invalid when handling interim responses, so
+		 * it's important to check out buffer remaining space.
+		 */
+		goto err_full;
+	}
 
 	b_reset(&outbuf);
 	outbuf = b_make(b_tail(res), b_contig_space(res), 0, 0);
 	/* Start the headers after frame type + length */
-	headers_buf = b_make(b_head(res) + 5, b_size(res) - 5, 0, 0);
+	headers_buf = b_make(b_tail(res) + 5, b_contig_space(res) - 5, 0, 0);
 
 	TRACE_DATA("encoding HEADERS frame", H3_EV_TX_FRAME|H3_EV_TX_HDR,
 	           qcs->qcc->conn, qcs);