How do I count how many checkboxes are selected on a page with jQuery

I want to count how many checkboxes on my page are selected using jQuery. I've written the following code:

      var numberOfCheckboxesSelected = 0;
      $(':checkbox').each(function(checkbox) {
          if (checkbox.attr('checked'))
              numberOfCheckboxesSelected++;
      });

However, my code errors out with the message "Object doesn't support this property or method" on the third line.

How do I count how many checkboxes are selected on my page?