The turn tracker allows arbitrary text to be entered into the initiative text box. Obviously this isn't as intended as initiative is usually a number. However, the box takes it, displays it, and saves it between sessions correctly. But when clicked on to edit, the text in the box is replaced with NaN (since it was expecting a number and that's not what it had).
Since the Turn Tracker does not care what it displays and has no rules associated with it, may I suggest you remove the expectation of a number? (It would be easier than making sure it is a number) :)