diff --git a/include/haproxy/buf.h b/include/haproxy/buf.h
index 78409c0..de359f5 100644
--- a/include/haproxy/buf.h
+++ b/include/haproxy/buf.h
@@ -519,6 +519,8 @@
 {
 	size_t half = b_contig_space(b);
 
+	BUG_ON(b_data(b) + len > b_size(b));
+
 	if (half > len)
 		half = len;