title: Unknown web method. Parameter name: methodName
body: <p>In researching this problem most SO issues were about the <code>static</code> method as a fix.</p>
  
  <p>Since it's not working with the real (and a bit sophisticated) WebMethod I've just created a simple one for the sake of checking if reaching the method itself is possible.</p>
  
  <pre><code>[WebMethod]
  [ScriptMethod(UseHttpGet = false)]
  public static string HelloWorld()
  {
      return "Hello World!";
  }
  </code></pre>
  
  <p>The call.</p>
  
  <pre><code>&lt;script&gt;
      $(document).ready(function () {
          $.ajax({
              type: "POST",
              contentType: "application/json; charset=utf-8",
              url: "usersWebMethods.aspx/HelloWorld",
              dataType: "json",
              success: function (data) {
                  alert(data.d);
              }
          });
     });
  &lt;/script&gt;
  </code></pre>
  
  <p>It always comes down to <code>500 (Internal Server Error)</code></p>
  
  <pre><code>Unknown web method HelloWorld.
  Parameter name: methodName
  Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 
  Exception Details: System.ArgumentException: Unknown web method HelloWorld.
  Parameter name: methodName
  </code></pre>
  
  <p>Why is this failing?</p>