NPE in SlidingLengthWindow.assertFact when some event in window expired

package org.drools;

import java.util.List

global List timeResults;
global List lengthResults;

declare OrderEvent
    @role( event )
    //@expires( 2m )
end

rule "collect with time window"
when
    $list : List( empty == false ) from collect(
              $o : OrderEvent() over window:time(30s) )
then
    timeResults.add( $list.size() );
end

rule "collect with length window"
when
    $list : List( empty == false ) from collect(
              $o : OrderEvent() over window:length(3) )
then
    lengthResults.add( $list.size() );
end