Have you added any custom Javascript to a script tagged passage in your story project, if so what does it do?
Otherwise we will need more information, like:
a. What exactly does the error message and any associated stack trace say?
b. what brand and version web-browser are you using to run the story HTML file.
c. what operating system (brand and version) are you running the web-browser on.
d. anything else related to your project that you think might be relevant....