[Error: [webdrivercss("screenshot

[Ryan.TLY]

var webdriverio = require('webdriverio');
var client = webdriverio.remote(options);
require('webdrivercss').init(client,{
screenshotRoot: xpath + '/' + time
});
client
.init()
.url(url)
.setViewportSize({
width: 1280,
height: 800
})
.webdrivercss('screenshot',[
{
name: 'screenshot',
elem: 'body'
}
], function(err, res) {
console.log(err);
})
.end();

{ [Error: [webdrivercss("screenshot",[])] <=
[saveDocumentScreenshot("/workspace/test_webd/tieba.baidu.com/L3AvMzUzMDM5Nzg2OD9fX3FhX3BhZ2VfZGlmZj0x/1441678124472/screenshot.png")] <=
Command failed: gm convert: Unable to open file (/workspace/test_webd/node_modules/webdrivercss/lib/../.tmp/0-1.png) [No such file or directory].
] code: 1, signal: null }

the screenshot.png did not be created, why?