Validation is not working in Webmatrix 2

My validation is not working. I dont know what i am missing, i read from here ,

@{
Validation.Add("number", Validator.Integer("Must be integer"));
     if(IsPost)
    {  
     if(Validation.IsValid())
     {
      <div>Number Submitted!</div>
     }
    }  
}
.......

  <form action="" method="post">
            <div>
            @Html.Label("Insert a number:", "number")
            @Html.TextBox("number",Request["number"])
            @Html.ValidationMessage("number")
            </div>
  <div>
  <input type="submit" value="submit">
  </div>
  </form>

I can type any value, and validation message shows nothing, am i missing something?

Thanks.

You should also add a Required validator:

Validation.Add("number", 
    Validator.Required("Gimme something, please?"),
    Validator.Integer("Must be integer")
);