Nested complexTypes in XSD returning error: Element content is not allowed, because the type definition is simple

I'm building an xml schema for this xml:

<?xml version="1.0" encoding="UTF-8"?>

<groups xmlns:xsi="http://ift.tt/ra1lAU" 
    xsi:noNamespaceSchemaLocation="basic.xsd">
<!--Comienza la descripción de los grupos sencillos-->
    <!--modificador-->
    <group tagName="A">
      <name>modificador</name>
      <type>simple</type>
      <words>
        <word>el<word>
        <word>este<word>
      </words>
    </group>
    <!--prefijos-->
    <group tagName="C">
      <name>prefijos</name>
      <type>simple</type>
      <words>
        <word>pre</word>
        <word>ante</word>
        <word>anti</word>
        <word>pro</word>
        <word>tri</word>
      </words>
    </group>
    <!--Composiciones de intérvalos-->
</groups>

And this is my xsd file:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://ift.tt/tphNwY">

<!-- definition of simple elements -->
<xs:element name="name" type="xs:string"/>
<xs:element name="type" type="xs:string"/>
<xs:element name="word" type="xs:string"/> 

<!-- definition of attributes -->
<xs:attribute name="tagName" type="xs:ID"/>

<!-- definition of complex elements -->
<xs:element name="words">
  <xs:complexType>
    <xs:sequence>
      <xs:element ref="word" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>
</xs:element>

<xs:element name="group">
  <xs:complexType>
    <xs:sequence>
      <xs:element ref="name"/>
      <xs:element ref="type"/>
      <xs:element ref="words" />
    </xs:sequence>
    <xs:attribute ref="tagName" use="required"/>
  </xs:complexType>
</xs:element>

<xs:element name="groups">
  <xs:complexType>
    <xs:sequence>
      <xs:element ref="group" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>
</xs:element>
</xs:schema>

I'm using nokogiri to validate my xml, but I'm getting the following error:

Element 'word': Element content is not allowed, because the type definition is simple. XML not schema valid

Why should word be complex then? And how do I do that because to me it's just fine. Thanks in advanced.