CustomValidator ClientValidationFunction not firing: ASP.NET

Question

I have a custom validator that reads a listbox and determines if the ControlToValidate, which is a textbox, contains the same string as at least one element in the listbox. My validator is not firing. I have an alert to check if it does, and it never shows up. I have the proper key in my Web.Config for my validators as well. Validator:

<asp:CustomValidator ID="alreadyRegValidator"
                            runat="server"
                            ControlToValidate="txbClassNum"
                            ErrorMessage="Not Removed. Course is registered for."
                            ClientValidationFunction="reg_validator"
                            Display="Dynamic"
                            CssClass="warning" />

Function:

    <script type="text/javascript" language="javascript">
        function reg_validator(source, args) {
            let toRemove = args.value.toUpperCase();
            let removable = true;
            for (let i = 0; i < document.getElementById("lbxRegisteredClasses").children.length; i++) {
                let li = document.getElementById("lbxRegisteredClasses").children[i].value;
                let tokens = li.Text.split('-');
                if (tokens[0].toUpperCase() === toRemove) {
                    removable = false;
                    break;
                }
            }

            args.IsValid = removable;
            alert('test');
        }
    </script>

I have tried everything from rebuilding my project to copying the Microsoft .NET API to a T.