Widget Property XML - Microflow Return Types

I have the following property configured in a Widget (xml)

<property key="mfForData" type="microflow" required="true">
        <caption>Microflow (Data)</caption>
        <category>Data</category>
        <description>The microflow to execute for dataset items</description>
        <returnType type="Object" />
    </property>

The microflow I'm using returns a non persisted entity (it's just a dumb domain class with a 1 : * association to another dumb domain (items) class. The association is a bit irrelevant at the moment as I'd just like to get / see the parent entity let alone anything else.

Basically, I want to manipulate the parent domain class (once returned) in the widget. However, I get the following error on the widget (when bought in to the mendix project).

Package 'blah blah': Error(s) in widget XML file 'VisJS/VisJS.xml': Error in property 'mfForData': Property return type must specify an entity property.

Help - I'm at a bit of a loss looking at the XML standard for widgets trying to work out what I'm missing?