Value not updated after property changes on `DependencyObject` + `INotifyPropertyChanged`

[lostmsu]

I am binding to an object, that implements both DependencyObject and INotifyPropertyChanged. The property only has a getter.

This works and gets updated as usual:

<Run Text="{Binding PowerLimitMilliwatts, Mode=OneWay}"/>

but this does not get updated:

<Run Text="{calc:Binding PowerLimitMilliwatts/1000, Mode=OneWay}"/>