Error patching a [PROP]-type relation that satisfies type-checker constraints

[RieksJ]

What happened

I have the following code fragments:

sessionXForm :: SESSION * XForm [UNI]

CLASSIFY XFErfrecht ISA XForm
xfErfrechtNewPartyContactOK :: XFErfrecht * XFErfrecht [PROP]

[ ... stuff left out ... ]

INTERFACE "XFErfrecht": sessionXForm;I[XFErfrecht] cRud BOX <FORM>

[ ... more stuff left out ... ] 

         , "Toevoegen": V;"_SESSION";sessionXForm cRud BOX <PROPBUTTON> 
            [ label: TXT "OK"
            , property: xfErfrechtNewPartyContactOK cRUd
            ]

They compile as expected and result in a running prototype. Clicking the button Toevoegen results in the following error, which I had not expected to occur:

Analysis

The expression in front of the BOX has a target atom of type of sessionXForm, which is the concept XForm.
The property-expression in the PROPBUTTON is xfErfrechtNewPartyContactOK, of which the source (and target) concept is XFErfrecht. There is no type-error as XFErfrecht ISA XForm. Therefore, there should be no problem in populating the property.

Workaround

A workaround for this problem is to insert I[XFErfrecht] before the BOX, as follows:

         , "Toevoegen": V;"_SESSION";sessionXForm;I[XFErfrecht] cRud BOX <PROPBUTTON>