diff --git a/peer/control/ping.go b/peer/control/ping.go
index 8d0826e..7ea0bf5 100644
--- a/peer/control/ping.go
+++ b/peer/control/ping.go
@@ -37,6 +37,8 @@ type Ping struct {
 // buf[:Size]. Taking the buffer lets callers reuse one across sends; every
 // field is written unconditionally so a reused buffer needs no pre-zeroing.
 func (p Ping) Marshal(buf []byte) []byte {
+	_ = buf[Size-1] // Panic if buffer is too small.
+
 	buf[0] = version
 	binary.BigEndian.PutUint64(buf[1:9], uint64(p.PingTS))
 	if p.SrcV4.IsValid() {