EmbeddedDocumentField: cannot convert value of type <class '...'> to bson

I have the following setup:

class InnerClass(Document):
    field: StringField()

class OuterClass(Document):
    field: EmbeddedDocumentField(embedded_document_type=InnerClass)

When calling:
await OuterClass(field=InnerClass(field="test")).save()

I get:
cannot convert value of type <class 'scout.persist.domains.InnerClass'> to bson

What could be the problem?
I followed the documentation as listed here: http://motorengine.readthedocs.io/en/latest/modeling.html

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