Custom tag helper processing

Hi,

I have created a custom tag helper named "asp-disabled" with a target element set to "input" tag:

[HtmlTargetElement("input")]
public class AspDisabledTagHelper : TagHelper
{
    private const string AspDisabledAttributeName = "asp-disabled";

    [HtmlAttributeName(AspDisabledAttributeName)]
    public bool IsDisabled { set; get; }

    public override void Process(TagHelperContext context, TagHelperOutput output)
    {
        if (IsDisabled)
        {
            output.Attributes.Add(new TagHelperAttribute("disabled", "disabled"));
        }

        base.Process(context, output);
    }
}

Razor engine executes Process method for every encountered "input" tag, even if the latter doesn't have specified "asp-disabled" attribute. I'm wondering is this a correct behavior or am I missing something in tag declaration.