问在istio配置ssl证书，但服务端调用报错，请问证书配置的方式有误吗？

在istio配置ssl证书，但服务端调用报错,

kind: Gateway

apiVersion: networking.istio.io/v1alpha3

metadata:

name: xserver2-gateway-https

namespace: xserver2

selfLink: >-

/apis/networking.istio.io/v1alpha3/namespaces/xserver2/gateways/xserver2-gateway-https

uid: b5e84e46-0f1d-4518-955e-0ed9a59f9bbb

resourceVersion: '27091511'

generation: 3

creationTimestamp: '2020-01-13T09:09:58Z'

spec:

servers:

- hosts:

- api.xxxx.com

port:

name: https

number: 443

protocol: HTTPS

tls:

mode: SIMPLE

privateKey: /etc/certs/xxxxcom/key

serverCertificate: /etc/certs/xxxxcom/cert

selector:

istio: ingressgateway

调用接口报错：

unable to verify the first certificate',

type: 'system',

errno: 'UNABLE_TO_VERIFY_LEAF_SIGNATURE',

code: 'UNABLE_TO_VERIFY_LEAF_SIGNATURE' }

证书在myssl.com验证降级为B级。