Why is the AutoCompleteExtender firing the Page_Load event instead of the service method?

So I'm trying to implement the AutoCompleteExtender tool from the AJAX Control Toolkit.

The following is the implementation of the AutoCompleteExtender on my ASPX page:

<asp:TextBox runat="server" ID="CustomerTextBox" CssClass="form-control" /><asp:RequiredFieldValidator runat="server" ControlToValidate="CustomerTextBox"
    CssClass="text-danger" ErrorMessage="The Customer field is required." Display="None" /><ajaxToolkit:AutoCompleteExtender ID="CustomerAutoCompleteExtender" runat="server" TargetControlID="CustomerTextBox"
    MinimumPrefixLength="1" EnableCaching="true" CompletionSetCount="1" CompletionInterval="1000" 
    ServiceMethod="GetAllCustomerNames"></ajaxToolkit:AutoCompleteExtender>

This is the service method implemented in the code behind file:

[System.Web.Services.WebMethod]
[System.Web.Script.Services.ScriptMethod]
public string[] GetAllCustomerNames(string prefixText, int count)
{

    List<string> allCustomerNames = new List<string>();
    List<Customer> allCustomers = GetAllCustomers();

    foreach (Customer customer in allCustomers)
    {
        if (customer.CustomerName.Contains(prefixText))
        {
            allCustomerNames.Add(customer.CustomerName);
        }
    }

    return allCustomerNames.ToArray();
}

The problem I'm facing is that whenever I type a character into the text box the Page_Load event fires instead of theGetAllCustomerNames method. Could someone please help me find where I'm going wrong?

Additional info:

• I'm using Visual Studio 2013.
• This is a ASP.NET Web Form application running on .NET 4.5.
• I used the default style and template as when a new project is created and so a Master Page is being used.
• The ToolkitScriptManager is specified in the Master File and I have setEnablePageMethods property to true.

Thanks in advance!