[Solved ]Undefined method `file_format' for nil:NilClass

I followed this video by Mockaroo https://www.youtube.com/watch?v=H91_P_Zr2M0. I followed exactly what the video tell me to do, and created an API for testing.
But when I click on the link: https://my.api.mockaroo.com/users.json?key=bcc83090
I get the message error: "Undefined method `file_format' for nil:NilClass". How can I fix this problem?
Thank you.
Solution: I change the link name from Users.json to an unique name like myuniquestring421.fson fixed the problem