QtQuick: Unset required property in Singleton is not an error?

In my QML application I want to use exchangeable Singletons holding properties (as StyleSheets, so to speak).

To have some kind of "virutal base class" defining all required properties, I've created a base QtObject holding all property declarations using required property. This way – I expected – any unset property was reported as an error.

The attached example holds the "virtual base class" with two properties:

import QtQuick

QtObject {
  required property int thisIsSet
  required property int thisIsNotSet
}  

The Singleton implements the base object and should set both, but intentionally only set one:

pragma Singleton
import QtQuick

SingletonBase {
  // Set the first "required" property:
  thisIsSet: 99

  // Do NOT set the second "required" property:
  // thisIsNotSet: 11
} 

When using the Singleton, the unset required property does not produce an error:

...
    Text {
      text: "thisIsSet: " + Singleton.thisIsSet
    }
    Text {
      text: "thisIsNotSet: " + Singleton.thisIsNotSet
    }
...

Its value is "0":

thisIsSet: 99
thisIsNotSet: 0

Other types also use some default (i.e. color = "black").

Expected Result:

"thisIsNotSet" is reported similarly to:

QQmlApplicationEngine failed to create component
"Required property thisIsNotSet was not initialized"