HTML tidy linter: <button> in <span> errors

Question

I'm using tidy 5.2.0.

The following works fine and shows no tidy linting errors:

<button type  = "button" class = "btn btn-default">  
    <i class="glyphicon glyphicon-calendar"></i>  
</button>

However, if I place the button in a span tag, tidy shows 3 errors:

<span class="input-group-btn">
    <button type  = "button" class = "btn btn-default">  
        <i class="glyphicon glyphicon-calendar"></i>  
    </button>
</span>

Error 1: missing </button>
Error 2: inserting implicit <span>
Error 3: replacing unexpected button with </button>