 <p>Heroku runs a case-sensitive filesystem, whereas your OS may not be case-sensitive. Try changing that require to this instead:</p> <pre><code> require 'card' </code></pre> <hr> <p>As a general rule of thumb, most files within Ruby are required using lowercase letters with underscores serving as a word separator, like this:</p> <pre><code> require 'card_shuffler' </code></pre>