Using assert in a complexType with complexContent

Hi Folks,

I got an error with the assert element in this complexType:

    <xs:complexType name="authorType" mixed="true">
        <xs:complexContent>
            <xs:extension base="property">
                <xs:sequence>
                    <xs:element ref="Person" />
                </xs:sequence>
            </xs:extension>
        </xs:complexContent>
        <xs:assert test="not(normalize-space(string-join((.)/text(), '')))" />
    </xs:complexType>

The error message says:

   Element xs:assert cannot appear here: expected no more elements

I thought that it was legal to position the assert there? Apparently not. What is the proper way to do this?

/Roger