diff --git a/src/core/ngx_proxy_protocol.c b/src/core/ngx_proxy_protocol.c
index 212763647..8d3bd01d9 100644
--- a/src/core/ngx_proxy_protocol.c
+++ b/src/core/ngx_proxy_protocol.c
@@ -282,6 +282,8 @@ ngx_proxy_protocol_write(ngx_connection_t *c, u_char *buf, u_char *last)
     ngx_uint_t  port, lport;
 
     if (last - buf < NGX_PROXY_PROTOCOL_V1_MAX_HEADER) {
+        ngx_log_error(NGX_LOG_ALERT, c->log, 0,
+                      "too small buffer for PROXY protocol");
         return NULL;
     }