cvc-complex-type.2.4.a: Invalid content was found starting with element 'first_name'. One of '{first_name}' is expected

I built the following ('Person.xsd') XSD:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema version="1.0" xmlns:xs="http://ift.tt/tphNwY"
           targetNamespace="urn:person.com.test"
           xmlns="urn:person.com.test">
    <xs:element name="person" type="Person" />
    <xs:complexType name="Person">
        <xs:sequence>
                    <xs:sequence>
                        <xs:element name="first_name" type="xs:string" />
                        <xs:element name="last_name" type="xs:string"/>
                    </xs:sequence>
        </xs:sequence>
    </xs:complexType>   
</xs:schema>

And the following XML document ('Person.xml'):

<?xml version="1.0"?>
<person 
    xmlns="urn:person.com.test"
    xmlns:xsi="http://ift.tt/ra1lAU" 
    xsi:schemaLocation="urn:person.com.test person.xsd" >
    <first_name>Joe</first_name>
        <last_name>Bloggs</last_name>
</person>

But when I validate the XML (I'm using Netbeans 8.x , but other validators I have tried give very similar results); I get the following unhelpful message:

XML validation started.
Checking file:[...]/validator/src/main/resources/person.xml...
Referenced entity at "file:[...]/validator/src/main/resources/person.xsd".
cvc-complex-type.2.4.a: Invalid content was found starting with element 'first_name'. One of '{first_name}' is expected. [7] 
XML validation finished.

Which seems to be saying - you are using the wrong element name 'first_name', it should be 'first_name' ?????

I suspect this has something to do with namespaces, but I can't work out what is wrong in particular here.....