why can not use import module in xquery assersion

Why I can not use import module in xquery assersion? I use something like:
import module namespace elster = "http://www.elster.com" at "http://runtest/test/RegTest/FuncLibrary.xq";
And I got the error:
XQuery Match Assertion failed for path [declare namespace ns1="http://www.elster-eis.com/ImportExport"; import module namespace elster = "http://www.elster.com" at "http://runtest/test/RegTest/FuncLibrary.xq"; let $dr := for $xyz in //ns1:GetReadingsResponse/ns1:GetReadingsResult/ns1:AMRDEF return $xyz let $mr := for $x in $dr/ns1:MeterReadings where ($x/ns1:Meter/@MeterName='${Billing Rex Meters#MeterName}' and $x/ns1:Meter/@SerialNumber='${Billing Rex Meters#serialNumber}') return $x where (exists($mr)) return success ] : RuntimeException:java.lang.reflect.InvocationTargetException