RegEx for specified string length range : XSD attribute element

Question

I'm trying to restrict an attribute element of a schema to be between 3 and 20 characters long, but I'm getting an error saying my RegEx is invalid:

<xs:attribute name="name" use="required">
    <xs:simpleType>
        <xs:restriction base="xs:string">
            <xs:pattern value="[A-Za-Z]{3,20}" />
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>

Any idea what I'm doing incorrectly here? Specific error is "Range end code point is less than the start end code point"