NotSupportedError, The payment method is not supported

What am I missing in my checking for whether or not to offer Apple Pay on my website?

<script async crossorigin
        src="https://applepay.cdn-apple.com/jsapi/v1.1.0/apple-pay-sdk.js"
        ></script>
...

<style>
apple-pay-button {
    display: none;
}
</style>
...
            <apple-pay-button buttonstyle="black" type="plain" locale="en-US" onclick="startApplePay('${APPLE_PAY_MERCHANT_ID}','${paymentForm.amount}');"></apple-pay-button>

So, the button is not displayed by default. I only change the style to displayed if:

window.onload = function() {
    if (isApplePaySupported()) {
        document.querySelector("apple-pay-button").style.display = "inline-block";
    };
}

function isApplePaySupported() {
    return (window.PaymentRequest &&
            window.ApplePaySession &&
            ApplePaySession.canMakePayments() &&
            ApplePaySession.supportsVersion(applePayVersion));
}

Yet, once in a while a click comes through that tries to create a PaymentRequest with

    const applePayMethod = {
        "supportedMethods": "https://apple.com/apple-pay",
        "data": {
            "version": applePayVersion,
            "merchantIdentifier": merchantIdentifier,
            "merchantCapabilities": [
                "supports3DS"
            ],
            "supportedNetworks": [
                "amex",
                "discover",
                "masterCard",
                "visa"
            ],
            "countryCode": "US"
        }
    };

and results in:

NotSupportedError, The payment method is not supported

What else might be "not supported" in the request for this particular user/device/wallet? In particular, that could be known immediately when the PaymentRequest is created, but before any payment instrument from the wallet is selected?

And, is there anything I could detect before showing the button?

Or, is it even possible for the button to be clicked by some kind of automation, even if it's not displayed?