Creating a generic method in C

Question

I'm attempting to integrate several comparable techniques into a single generic method. I have a few methods that return the value of a querystring, or null if the querystring doesn't exist or isn't formatted properly. This would be simple if all of the types were natively nullable, however for numbers and dates, I have to use the nullable generic type.

Here's what I've got right now. However, if a numeric number is invalid, it will return a 0, which is regrettably an acceptable value in my cases. Is there anyone who can assist me? 

public static T GetQueryString<T>(string key) where T : IConvertible
{
    T result = default(T);

    if (String.IsNullOrEmpty(HttpContext.Current.Request.QueryString[key]) == false)
    {
        string value = HttpContext.Current.Request.QueryString[key];

        try
        {
            result = (T)Convert.ChangeType(value, typeof(T));  
        }
        catch
        {
            //Could not convert.  Pass back default value...
            result = default(T);
        }
    }

    return result;
}