How to use Conditional Type Assignment for different integer types in XSD

XSD1.1 allows the type of an element to depend on one of its attributes. For example,

<element kind='s'>100</element>

will cause the type of 'element' to be xs:short. Here is what I have got:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://ift.tt/tphNwY" 
           xmlns:vc="http://ift.tt/REsXlC"
           elementFormDefault="qualified" 
           attributeFormDefault="unqualified"
           vc:minVersion="1.1">
   <xs:complexType name="GenericInt">
      <xs:simpleContent>
         <xs:extension base="xs:integer">
            <xs:attribute name="kind" type="xs:string"/>
         </xs:extension>
      </xs:simpleContent>
   </xs:complexType>
   <xs:element name="integer" type="GenericInt">
      <xs:alternative test="@kind='b'" type="xs:byte"/>
      <xs:alternative test="@kind='s'" type="xs:short"/>
      <xs:alternative test="@kind='i'" type="xs:int"/>
      <xs:alternative test="@kind='l'" type="xs:long"/>
      <xs:alternative                  type="GenericInt"/>
   </xs:element>
</xs:schema>

When I tried to save the file in Altova XMLSpy, an error occurred: cos-st-derived-ok.2: Simple type definition 'xs:byte' is not validly derived from 'GenericInt'.

So how should I correct the XSD code?