pushDynamicScreen returns error

[adt2]

Trying to use an iconbutton '+' to open a modal (full screen) view. I can open the modal view just fine from a regular button on screens without the PersistentNavBar, but when I try...

IconButton(
            onPressed: () {
              PersistentNavBarNavigator.pushDynamicScreen(context,
                  screen: PageName(), withNavBar: false);
            },

...I get an error: "_CastError (type 'PageName' is not a subtype of type 'Route' in type case)". I'm not sure if I'm doing something wrong or if this is a bug.