Error validating HTML: The for attribute of the label element must refer to a form control

I don't know why I keep getting this error while checking my page at http://validator.w3.org/check The error was:

Line 46, Column 68: The for attribute of the label element must refer to a form control. 
<label class="environment-label" for="environment_form">Environments:</label>

I believe I provided an id reference for my label to the outer form, why it keep bugging me about this error?

<div>
    <form id="environment_form" method="post">
        <div class="styled-select">
            <label class="environment-label" for="environment_form">Environments:</label>
            <select name="environment_dropdown" onchange="selectionChanged()">
                <option @(ViewData["selection"] == null || string.IsNullOrEmpty(ViewData["selection"].ToString()) ? "selected" : "")>select one</option>
                @foreach (string name in Model) { 
                    <option @(ViewData["selection"] != null && ViewData["selection"].Equals(name) ? "selected" : "")> 
                        @name
                    </option>
                }
            </select> 
        </div>
    </form>
</div>