Where to place an XSD 1.1 assert?

Question

I'm trying unsuccessfully to use an xs:assert to validate based on the values of two attributes. I keep getting

s4s-elt-must-match.1: The content of data must match (annotation?, (simpleType | complexType)?, (unique | key | keyref)*)). A problem was found starting at: assert.

I've looked thru a bunch of questions. I even copied one of the answers given, but even it gives the same error. It's telling me the format of the test is invalid, but I can't find any example that does it differently. Can anyone tell me what I'm doing wrong?

TIA

Here's the answer I copied, with the error inline as a comment:

<?xml version="1.0" encoding="UTF-8" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:vc="http://www.w3.org/2007/XMLSchema-versioning"
  vc:minVersion="1.1">

  <xs:element name="data">
    <xs:complexType>
      <xs:attribute name="min" type="xs:int"/>
      <xs:attribute name="max" type="xs:int"/>
    </xs:complexType>
    <xs:assert test="@min le @max"/>
    <!--s4s-elt-must-match.1: The content of 'data' must match (annotation?, (simpleType | complexType)?, (unique | key | keyref)*)). A problem was found starting at: assert.-->
  </xs:element>

</xs:schema>

Edit: Here it is with the assert inside the complexType, showing a different error.

<?xml version="1.0" encoding="UTF-8" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:vc="http://www.w3.org/2007/XMLSchema-versioning"
  vc:minVersion="1.1">

    <xs:element name="data">
        <xs:complexType>
            <xs:attribute name="min" type="xs:int"/>
            <xs:attribute name="max" type="xs:int"/>
            <xs:assert test="@min le @max"/>    
<!-- s4s-elt-invalid-content.1: The content of '#AnonType_data' is invalid.  Element 'assert' is invalid, misplaced, or 
 occurs too often. -->                      
        </xs:complexType>
    </xs:element>

</xs:schema>

Answer 1

Possible issues:

1. Make sure your XSD processor supports XSD 1.1.
2. Move the xs:assert element to within xs:complex:

Change from

  <xs:element name="data">
    <xs:complexType>
      <xs:attribute name="min" type="xs:int"/>
      <xs:attribute name="max" type="xs:int"/>
    </xs:complexType>
    <xs:assert test="@min le @max"/>
  </xs:element>

to

  <xs:element name="data">
    <xs:complexType>
      <xs:attribute name="min" type="xs:int"/>
      <xs:attribute name="max" type="xs:int"/>
      <xs:assert test="@min le @max"/>
    </xs:complexType>
  </xs:element>