FactoryBuilderSupport exception messages could be improved in checkValueIsType() and checkValueIsTypeNotString().
Today the messages read: "The value of argument 'foo' must be of type Bar."
This doesn't tell the user what the input was that caused the failure.
A better message is: "The value of argument 'foo' must be of type Bar. Found: Baz"
This is an easy change to make, and cannot cause a NPE or anything like that.