为什么XSD说我的元素不完整？

I have an XSD of this form:

我有这种形式的XSD：

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="https://www.w3.org/2001/XMLSchema"
    targetNamespace="https://www.example.org/example"
    xmlns:tns="https://www.example.org/example" elementFormDefault="qualified">

    <complexType name="bType">
    </complexType>

    <complexType name="aType">
        <choice maxOccurs="unbounded">
            <element name="a" type="tns:aType" />
            <element name="b" type="tns:bType" />
        </choice>
    </complexType>

    <element name="topelement">
        <complexType>
            <sequence>
                <element name="a" type="tns:aType" maxOccurs="1" />
            </sequence>
        </complexType>
    </element>
</schema>
<?x