grpc.credentials.createSsl(root_cert) 导致错误：14 不可用：grpc-node.js 中未建立连接

Question

var root_cert = readFileSync(__dirname + 
'/credentials/localhosttest.cer'); 
var client = new PSC_PatientProto.service(target,grpc.credentials.createSsl(root_cert)); 

上面的代码导致以下错误

error in call

Error: 14 UNAVAILABLE: No connection established at 
Object.callErrorFromStatus (C:\acp-electron-shell\electron- 
app\node_modules\@grpc\grpc-js\build\src\call.js:31:26) at 
Object.onReceiveStatus (C:\acp-electron-shell\electron- 
app\node_modules\@grpc\grpc-js\build\src\client.js:180:52)