getElementById Object doesn't support this property or method

代码：

master_headertop =document.getElementById('master_headertop');
if (master_headertop)
      master_headertop.style.display = 'none';

错误提示：

Object doesn't support this property or method.

更正为：

var master_headertop =document.getElementById('master_headertop');
if (master_headertop)
        master_headertop.style.display = 'none';

以下也是错误的：

getElementById('master_headertop').style.display = 'none';