diff --git a/src/ngx_http_lua_kong_ssl.c b/src/ngx_http_lua_kong_ssl.c
index 52d0b012..3a83ed53 100644
--- a/src/ngx_http_lua_kong_ssl.c
+++ b/src/ngx_http_lua_kong_ssl.c
@@ -380,7 +380,7 @@ ngx_http_lua_kong_set_upstream_ssl(ngx_http_request_t *r, ngx_connection_t *c)
 
 
 static X509 *
-ngx_http_lua_kong_x509_copy(X509 *in)
+ngx_http_lua_kong_x509_copy(const X509 *in)
 {
     return X509_up_ref(in) == 0 ? NULL : in;
 }