"Data type not defined" Error when Loading Dataset: Please HELP!

[Ganesh Muthuvelu]

Hello,
What is wrong with this simple schema. When I try to load it to a dataset it
gives me an error, "Data type not defined"

Thanks.

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<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:tns="http://tempuri.org"
targetNamespace="http://tempuri.org"
xmlns:xs="http://www.w3.org/2001/XMLSchema">

<xs:element name="Document" type="tns:OMessage">
</xs:element>

<xs:complexType name="OMessage">
<xs:sequence>
<xs:element minOccurs="1" maxOccurs="1" name="sessionId"
type="xs:normalizedString">
</xs:element>
<xs:element minOccurs="1" maxOccurs="15" name="command"
type="tns:OCommand">
</xs:element>
</xs:sequence>
</xs:complexType>

<xs:complexType name="OCommand" abstract="true">
<xs:sequence>
</xs:sequence>
<xs:attribute name="echo" type="xs:string" use="optional">
</xs:attribute>
</xs:complexType>
</xs:schema>

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