publish pod repo encounter "xcodebuild: Returned an unsuccessful exit code"

I'm trying to publish a pod repo writing in Objective-C to public space. It seems everything works well, but when I use "pod lib lint" to check the podspec file, following error appeared:

- ERROR | [iOS] xcodebuild: Returned an unsuccessful exit code. You can use `--verbose` for more information.
- NOTE  | [iOS] xcodebuild:  xcodebuild: error: 'App.xcworkspace' does not exist.**

Pod::Spec.new do |s|
  s.name     = 'ImageRightButton'
  s.version  = '0.0.3'
  s.platform = :ios, '7.0'
  s.license  = 'MIT'
  s.summary  = 'Give a button with titlelabel in left and image in right'
  s.homepage = 'https://github.com/fizix100/ImageRightButton'
  s.author   = { 'fizix100' => 'fizix100@hotmail.com' }
  s.source   = { :git => 'https://github.com/fizix100/ImageRightButton.git',    :tag => s.version.to_s }

  s.description = 'Inherited from UIButton, the button has a BOOL property "isImageRight",
         when set this property as YES, the button can set its title in left and
         image in right'

  s.prefix_header_contents = '#import <UIKit/UIKit.h>'
  s.source_files = 'Classes/*.{h,m}'
  s.preserve_paths  = 'Example'
  s.requires_arc = true
end**

I'm using:

CocoaPods 1.1.0.beta.2

Ruby ruby 2.0.0p648

Xcode 8.0

OS X 10.11.6

I tried the other published podspec files(as SVPullToRefresh) with "pod spec lint", they showed the same error, so I don't think this error was caused from my spec's format. This problem has waste me a full day, still unsolved.

Can anyone help me on this?

This problem has been resolved, just due to cocoapods version 1.0.1。 Today after my pods was updated to version 1.1.0, the error disappeared, my repo has been published, a very small Control inherited from UIButton that can show button.image on the right and button.title on the left github link

Meantime, I found that the pod command "pod lib lint" did show a wrong error to me:

- ERROR | [iOS] file patterns: The 'source_files' pattern did not match any file.

but command "pod spec lint" passed and I successfully published it. The conclusion was that "pod lib lint" went error, it just misguide me.