Effect maker: Exported effects with images do not work when applied to project

When I try to use an exported effect that contains image, such as opacity mask, in the project. I get this error:

Warning: file:///C:/dev/qt/DesignStudio/apps/UntitledProject81/asset_imports/Effects/OpEff/OpEff.qml:24:13: Unable to assign QQuickImage to QUrl (file:///C:/dev/qt/DesignStudio/apps/UntitledProject81/asset_imports/Effects/OpEff/OpEff.qml:24, (null))

To repro:

• Create new effect using opacity mask node
• Export effect
• Add a Rectangle node to scene
• Drag the effect under the rectangle node

-> The effect doesn't work. Puppet trace shows the above warning.