"Is this a bug in R2014b?"
Nope. It is because you defined a variable named mean.
"Is the workaround to write my own mean function?"
Nope. The solution is to delete the variable named mean.
This has been discussed many times before:
etc. etc.
Tip for the future: whenever you have a problem with a function one of the first things you should do is check it using which.
