ScriptRunner fails to implement Liskov principle?

[Roman Kharytonov]

Because issuePicker custom field is not parseable (does not support IN or IN (subquery) ) by search I'm trying to implement own JQL search plugin to run something like this:

project = MYPROJECT and issuetype = Epic and issueFunction in issuePickerSearch("Project Link", "filter=12345")

Which should return all Epics whose "Project Link" is on the list of the issues returned by filter

The code itself works fine, but the problem is at the very end after getQuery method of my issuePickerSearch plugin returns the results to Jira:

public Query getQuery(QueryCreationContext queryCreationContext, FunctionOperand operand, TerminalClause terminalClause) {

    BooleanQuery.Builder boolQueryBuilder = new BooleanQuery.Builder();

    String searchField = operand.getArgs().get(0);
    CustomField customField = customFieldManager.getCustomFieldObjectByName(searchField);

    SearchService.ParseResult searchByFilter = searchService
        .parseQuery(user, operand.getArgs().get(1));

    if (searchByFilter.isValid()) {
        SearchResults resultIssuesInFilter;

        try {
            List<Issue> allIssuesWithCustomField = getAllIssuesWithCustomField(customField);
            resultIssuesInFilter = searchService
                .search(user, searchByFilter.getQuery(), PagerFilter.getUnlimitedFilter());

            List<String> matchingParentKeys = resultIssuesInFilter.getIssues().stream()
                .map(Issue::getKey)
                .collect(Collectors.toList());


            List<Issue> foundIssues = allIssuesWithCustomField.stream()
                .filter(i -> hasParentInFilter(i, matchingParentKeys, customField))
                .collect(Collectors.toList());


            BooleanQuery.setMaxClauseCount(foundIssues.size());

            foundIssues.forEach(issue -> boolQueryBuilder
                .add(new TermQuery(new Term("issue_id", issue.getId().toString())),
                    Occur.SHOULD));


        } catch (SearchException | NullPointerException e) {
            log.error("Major search error: {}", e);
            return null;
        }

    }

    return boolQueryBuilder.build();
}

The code above throws     

org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object  with class 'org.apache.lucene.search.BooleanQuery' to class 'org.apache.lucene.search.Query'

How come? BooleanQuery extends Query and according to Liskov's principle should be perfectly compatible

Jira 7.2.9

Scriptrunner  5.5.9