Static error while running the Query 15 in test suite

Query 15 does not run as a part of the XMark Test suite. It gives the following error:
It is a static error if an expression refers to an element name, attribute name, schema type name, namespace prefix, or variable name that is not defined in the static context, except for an ElementName in an ElementTest or an AttributeName in an AttributeTest.
at org.apache.vxquery.xmlquery.translator.XMLQueryTranslator.translateVarRefNode(XMLQueryTranslator.java:1138)

However, the query runs when run independently.