Invalid error when referring an attribute from another namespace in an overridden schema

If I validate the "override.xsd" schema with Xerces I get an error. It seems that it cannot resolve the "attr" attribute that is declared in an imported schema.
I tested on the xml-schema-1.1-dev branch.

---- override.xsd------------------
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:imp="importNs" targetNamespace="mainNs">

<xs:import namespace="importNs" schemaLocation="import.xsd"/>

<xs:override schemaLocation="extended.xsd">
<xs:element name="mainElem">
<xs:complexType>
<xs:attribute ref="imp:attr"/>
</xs:complexType>
</xs:element>
</xs:override>
</xs:schema>
--------------------------------------

----------- extended.xsd ----------------
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:imp="importNs" targetNamespace="mainNs">
<xs:element name="mainElem">
<xs:complexType>
<xs:attribute name="myAttr" type="xs:Name"/>
</xs:complexType>
</xs:element>
</xs:schema>
---------------------------------------------

------- import.xsd-----------------------
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="importNs">
<xs:attribute name="attr" type="xs:string"/>
</xs:schema>
--------------------------------------------

-------- Error-------------------------------------------
[Error] :-1:-1: src-resolve.4.2: Error resolving component 'imp:attr'. It was detected that 'imp:attr' is in namespace 'importNs', but components from this namespace are not referenceable from schema document 'null'. If this is the incorrect namespace, perhaps the prefix of 'imp:attr' needs to be changed. If this is the correct namespace, then an appropriate 'import' tag should be added to 'null'.
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