使用github API创建新分支时获取“引用更新失败”

Question

我正在尝试通过Github API创建一个关闭master的新分支，但是我收到了“Reference update failed”错误。这是我的红宝石代码：

def self.create_ref access_token, github_user, github_repo, ref
    sha=nil
    url = "https://api.github.com/repos/#{github_user}/#{github_repo}/git/refs/heads/master"
RestClient.get(url,{params:{access_token:access_token},accept:'json'}){ |response, request, result| 
       puts response      
       sha=JSON.parse(response)['object']['sha']
    }  
    url = "https://api.github.com/repos/#{github_user}/#{github_repo}/git/refs"
    RestClient.post(url,{ref:name,sha:sha }.to_json,{params:{access_token:access_token},accept:'json'}){ |response, request, result| 
       puts response
}    
end

使用master sha代码是否有错误的方法来创建新的ref？