Access ASP:Text Value with Watin

I've only started working with watin a few days ago and I'm having a slight problem. I am writing a very simple test to change the text of UserName. The code is:

Markup

<div class="col-md-10">
<asp:TextBox runat="server" autocomplete="off" ID="UserName" CssClass="form-control" placeholder="Username" />
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="UserName"
CssClass="text-danger" ErrorMessage="The Username field is required." />
</div>

And my Test code is:

[TestClass]
public class TestClass
{
    [TestMethod]
    public void TestMethod1()
    {
        using (var browser = new IE("My Url"))
        {
            browser.TextField(Find.ByName("UserName")).TypeText("some text");
        }
    }
}

When ever I run the test I keep getting the following error:

Could not find INPUT (hidden) or INPUT (password) or INPUT (text) or INPUT (textarea) or TEXTAREA element tag matching criteria: Attribute 'name' equals 'UserName' at about:blank

I've come across this question but still get the same error. Can someone please tell me where I'm going wrong. Thanks in advance for all your help and support.