How to get data in standard error output using AppleScript shell

I have the following function:

func executeCommandAsAdminWithAppleScript(command: String, inout output: String?, inout errorDesc: String?) -> Bool {
    var errorInfo: NSDictionary?
    let source = "do shell script \"\(command)\" with administrator privileges"
    let script = NSAppleScript(source: source)
    let eventResult = script?.executeAndReturnError(&errorInfo)

    if eventResult == nil {
        errorDesc = nil
        if let code = errorInfo?.valueForKey(NSAppleScriptErrorNumber) as? NSNumber {
            if code.intValue == -128 {
                errorDesc = "The administrator password is required to do this.";
            }

            if errorDesc == nil {
                if let message = errorInfo?.valueForKey(NSAppleScriptErrorMessage) as? NSString {
                    errorDesc = String(message)
                }
            }
        }
        return false
    } else {
        output = (eventResult?.stringValue)!
        return true
    }
}

This function basically runs a command with admin privileges. Now I try to execute commands like echo Hello World, the output parameter does have a string that looks like Hello World, which is outputted into stdout. But when I tried to execute commands that output data into stderr, the output cannot retrieve the data. So is there a way to retrieve data in stderr without using NSTask? Thanks.