Abstract type and derivation

Hi,

I write a XML Schema using the "abstract type" feature, and related extension .
Every thing is ok with XSV, but I am not able to load a document with XML Spy
(3.5 beta2) because of it raise an error on an element using abstrcat type.
Error message is 
    Nothing allowed inside empty element "blabla"

Is it a problem in my declarations or a mistake in XMLSpy ?

(details below)

Regards

-- bruno --
----------------------------------------
Bruno Chatel
Tel : (+33)[0] 4 96 11 14 57
Email : bcha@chadocs.com
http://www.chadocs.com
----------------------------------------


mc.xsd
--------
<xsd:element name="then" type="actType"/>
<xsd:complexType name="actType" abstract="true"/>

phr.xsd
--------
<import namespace="..." schemaLocation="mc.xsd/>
 <xsd:complexType name="phrActType">
  <xsd:complexContent>
   <xsd:extension base="mc:actType">
      <xsd:choice maxOccurs="unbounded">
        <!-- elements -->
        ...
      </xsd:choice>
   <xsd:extension>
 </xsd:complexContent>
</xsd:complexType>


phr.xml (corresponding to phr.xsd)
------------------------------------
<mc:then xsi:type="phrActType">
    .....
</mc:then>


XMLSpy error message
-------------------------
Nothing allowed inside empty element "mc:then"