api.js "TypeError: S is undefined" when using XHTML

[openwebsolns]

Summary

When rendering a web page using the standard Content-Type header with value application/xhtml+xml; charset=UTF-8, the web console displays error message:

Uncaught TypeError: K is undefined
    h https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:270
    P https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:176
    <anonymous> https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:720
    T https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:102
    P https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:190
    h https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:264
    VA https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:656
    G https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:14
    m https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:53
    m https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:53
    E https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:376
    m https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:56
    T https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:165
    m https://www.gstatic.com/recaptcha/releases/M-QqaF9xk6BpjLH22uHZRhXt/recaptcha__en.js:56

Although at other times it claimed S is undefined instead.

Steps to reproduce

Generate a valid XHTML page. The only difference between working and error above is the HTTP header from the server changing:

• (works) text/html; charset=UTF-8
• (fails) application/xhtml+xml; charset=UTF-8