Please describe the problem:
The xml document below is not validating correctly, I get the follow error:
complex type 'pwdcontrol_type': The content type must specify a particle.

With xerces-j the document is validated correctly.

-----------XML document---------------

<?xml version="1.0" encoding="UTF-8"?>
<policy xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:noNamespaceSchemaLocation="inheritance_test1.xsd">

    <pwdcontrol min_chars="dgsd">
        
    </pwdcontrol>
</policy>

------------- XML SCHEMA ------------------

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:element name="policy">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="control" maxOccurs="unbounded">
                    
                </xs:element>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
    <xs:element name="control" abstract="true" type="control_type">
    </xs:element>
        
    <xs:element name="loggingcontrol" type="loggingcontrol_type" substitutionGroup="control">
        
    </xs:element>
    <xs:complexType name="control_type" abstract="true">
        <xs:simpleContent>
            <xs:extension base="xs:string">
                
            </xs:extension>
        </xs:simpleContent>
    </xs:complexType>
    
    
    <xs:complexType name="pwdcontrol_type">
        <xs:complexContent>
            <xs:extension base="control_type">
                <xs:attribute name="min_chars" use="required"/>                  
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
    
    <xs:complexType name="loggingcontrol_type">
        <xs:complexContent>
            <xs:extension base="control_type">
                <xs:attribute use="required" name="logfile">
                    
                </xs:attribute>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
    <xs:element name="pwdcontrol" substitutionGroup="control" type="pwdcontrol_type">
        
    </xs:element>
</xs:schema>





Steps to reproduce:
1. Put xml document above into a file
2. Put the schema in a file in the same directory named inheritance_test1.xsd
3. Validate using libxml2


Actual results:
I get the error:
complex type 'pwdcontrol_type': The content type must specify a particle.

Expected results:
Correct validation.

Does this happen every time?
Yes.

Other information: