This is a strange one! Can you try this with a fresh/new R session, using the "demo" Shiny app that you get when navigating to File -> New -> Shiny Web App in the RStudio IDE? It might be worth clearing your environment, too. The little "broom" icon in the Environment pane can help you do that. If you have a spurious function defined, that could cause problems.
It's hard for us to help without a bit more clarifying information / examples 