ASP.NET PageMethod onSuccess function returns a HTML String
<p>This is weird. My onSuccess() function in the javascript file returns a massive <strong>HTML Code of my entire web page</strong> instead of the value I actually return from my C# WebMethod</p>

<blockquote>
  <p>1) I am running Visual Studio 2015 on Windows 10</p>
  
  <p>2) When debugged, I kept a breakpoint at the SignupAccount C# method. The breakpoint was not hit. It didn't get to that point. BUT the onSuccess function was called</p>
</blockquote>

<p>Here's an explanation:</p>

<p><strong>ASP.NET Web Form's HTML:</strong></p>

<pre><code>&lt;html xmlns="http://www.w3.org/1999/xhtml"&gt;
 &lt;head runat="server"&gt;
   &lt;!--Code--&gt;
 &lt;/head&gt;
 &lt;body class="account2 signup" data-page="signup"&gt;
   &lt;!--Code--&gt;
   &lt;form id="form1" runat="server"&gt;
        &lt;asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true"&gt;&lt;/asp:ScriptManager&gt;

    &lt;input type="text" name="firstname" id="firstname" placeholder="First Name" required autofocus&gt;

    &lt;input type="text" name="lastname" id="lastname" placeholder="Last Name" required autofocus&gt;

    &lt;button type="submit" onclick="signup()" id="submit-form" class="btn btn-lg btn-dark btn-rounded" data-style="expand-left"&gt;Sign In&lt;/button&gt;

    &lt;script src="JS/Account.js"&gt;&lt;/script&gt;
   &lt;/form&gt;
 &lt;/body&gt;
&lt;/html&gt;
</code></pre>

<p><strong>WebMethod in C# Code Behind:</strong></p>

<pre><code>[WebMethod]
public static string SignupAccount(string fname, string lname)
{
    return "OK";
}
</code></pre>

<p><strong>Javascript:</strong></p>

<pre><code>function signup() {
    var fname = document.getElementById("firstname").value;
    var lname = document.getElementById("lastname").value;

    PageMethods.SignupAccount(fname, lname, onSuccess, onFailure);

    function onSuccess(val) {
       alert(val);
       //Technically, this should display "OK". But instead it displays a HTML String 
       //of my entire Web Page starting from the &lt;html&gt; tag to the end
    }

    function onFailure() {}
}
</code></pre>

<p>Why is it happening? I believe the procedure and the code is correct. Is this something to do with Visual Studio?</p>

<p><strong>EDIT</strong></p>

<p>This is the response I get from the onSuccess function</p>

<p><a href="http://pastebin.com/6MjAFPY9" rel="nofollow">http://pastebin.com/6MjAFPY9</a></p>