问启动appium失败: TypeError

EN

error: Unhandled error: TypeError: Path must be a string. Received null
    at assertPath (path.js:9:11)
    at Object.posix.resolve (path.js:427:5)
    at [object Object].<anonymous> (/usr/local/lib/node_modules/appium/lib/devices/android/android-common.js:686:29)
    at /usr/local/lib/node_modules/appium/node_modules/appium-adb/lib/adb.js:277:7
    at ChildProcess.exithandler (child_process.js:205:7)
    at emitTwo (events.js:88:13)
    at ChildProcess.emit (events.js:173:7)
    at maybeClose (internal/child_process.js:819:16)
    at Socket.<anonymous> (internal/child_process.js:320:11)
    at emitOne (events.js:78:13)
    at Socket.emit (events.js:170:7)
    at Pipe._onclose (net.js:470:12) context: [POST /wd/hub/session {"desiredCapabilities":{"app":"/Users/user/Documents/app.apk","appPackage":"package","appium-version":"1.0","platformVersion":"5.1","automationName":"Appium"]

DesiredCapabilities capabilities = new DesiredCapabilities();
        capabilities.setCapability("appium-version", "1.0");
        capabilities.setCapability("automationName", "Appium");
        capabilities.setCapability("platformName", "Android");
        capabilities.setCapability("platformVersion", "5.1");
        capabilities.setCapability("deviceName", "device");
        capabilities.setCapability("app", "/Users/user/Documents/app.apk");
        capabilities.setCapability("appPackage", "package");
        WebDriver wd = new AndroidDriver(new URL("http://127.0.0.1:4444/wd/hub"), capabilities);
        wd.manage().timeouts().implicitlyWait(60, TimeUnit.SECONDS);
        wd.close();