Pedro pointed out that I had some incomplete clock entries in my Emacs configuration. org-resolve-clocks prompts you for what to do with each open clock entry in your Org agenda files and whatever Org Mode files you have open.

If you don't feel like cancelling each clock with C, I also wrote this function to delete all open clocks in the current file.

(defun my-org-delete-open-clocks ()
  (interactive)
  (flush-lines
   (rx
    line-start
    (zero-or-more space)
    "CLOCK:"
    (one-or-more space)
    (regexp org-ts-regexp-inactive)
    (zero-or-more space)
    line-end)))