Malformed `<.simple_form>` in live view produces wierd error message

I added following live view template from scratch.
Please be noted that it has bad <:action> slot tag (should be <:actions> in plural form

<.header>
    새봄맞이 대 할인 이벤트!
    <:subtitle> 10% 싸게 해줄께요~ </:subtitle>
</.header>

<div>
    <.simple_form
        for={@form}
        id="promo-form"
        phx-change="validate"
        phx-submit="save"

    >
        <.input field={@form[:first_name]} type="text" label="First Name" />
        <.input field={@form[:email]} type="email" label="Email" />

        <:action>
            <.button>신청하기</.button>
        </:action>

    </.simple_form>
</div>

Above template produce following error message

[error] GenServer #PID<0.3845.0> terminating
** (FunctionClauseError) no function clause matching in Plug.CSRFProtection.get_csrf_token_for/1
...

That error message made me have hard time in fixing typo in template.

Why such error message is displayed?