Presence of tasks file breaks pages that use the add-on's plugin

I'm working on a task for an add-on, but with the file in place, I get an error when I load pages that use the plugin:

In Chrome:

No data received ERR_EMPTY_RESPONSE

In Firefox:

Server not found

If I delete the file, there's no problem.

• The add-on is called Tiger,
• the filename is tasks.tiger.php, and

• the file's contents are:

  <?php
  
  class Tasks_tiger extends Tasks
  {
      public function define()
      {
          $this->add(60, 'create');
      }
  
  
  
  
  
      public function create()
      {
          ...
      }
  }
  

I tried:

• Linting the PHP,
• removing everything except the class definition,
• removing everything except the class definition and define() method, and
• dropping in Raven's tasks file and renaming the file and classname, and removing the Finder namespace.

What else can I try?