Java EE (Java Enterprise Edition) General Discussion

I have a form to upload files and everything work as expected.
The problem is when I submit the form and the file field is empty.
This is the error i get:

javax.servlet.ServletException: BeanUtils.populate
... [stack trace] ...

root cause
java.lang.IllegalArgumentException: argument type mismatch
... [stack trace] ...

The file must be an optional field, even though i haven't done the validation yet, so all fields can be left blank.
I don't know how to fix this problem.