XML validation - element not declared [duplicate]

I don't get this xml:

<mappings>
 <mapping>
  <protobufMessage>pb::Header</protobufMessage>
  <rosPackage>std_msgs</rosPackage>
  <rosMessage>Header</rosMessage>
 </mapping>
</mappings>

validated against this schema, which is defined inline in the source code:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified"
>
<xs:element name="mappings">
    <xs:complexType>
        <xs:sequence>
                <xs:element name="mapping" maxOccurs="unbounded">
                    <xs:complexType>
                        <xs:sequence>
                            <xs:element name="protobufMessage" type="xs:string"/>
                            <xs:element name="rosPackage" type="xs:string"/>
                            <xs:element name="rosMessage" type="xs:string"/>
                        </xs:sequence>
                    </xs:complexType>
                </xs:element>
        </xs:sequence>
    </xs:complexType>
</xs:element>

</xs:schema>

I get the error message

org.xml.sax.SAXParseException; cvc-elt.1: Cannot find the declaration of element 'mappings'. at [...]

The code for validation (xtend code):

def private validate(InputStream is) {
    val parserF = DocumentBuilderFactory.newInstance
    val parser = parserF.newDocumentBuilder 
    val document = parser.parse(is)
    val validatorF = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI)
    val schemaSource = new StreamSource(new StringReader(schema)) // schema is a String member which contains the schema
    val valSchema = validatorF.newSchema(schemaSource)
    val validator = valSchema.newValidator
    validator.validate(new DOMSource(document))
}

What am I doing wrong here?