Unable to set pinMode to INPUT_PULLUP.

When trying to set the input mode of a pin to "INPUT_PULLUP", strange behavior occurs.
In this case, we have the input connected to a simple toggle-switch, which, if PULLUP is set, always returns a LOW.

Looking through the prototype sketch, there seems to never be a reference to PULLUP, which might explain why it seems to be impossible to set the input type to PULLUP.