Steps to reproduce

1. run the project https://github.com/hawkchen/zssperformance with zss 3.5.0 (so you have to change the zss-version in pom.xml accordingly)
2. use Jmeter 2.10 to open test plan CollaborationEditing.jmx (the file is under [PROJECT-BASE]\TestPlan)(original test plan https://docs.google.com/a/potix.com/document/d/1pK-mi4hO14mKkmlf1qZLwi-wp3uU04wcMVzHk6MuS5I/edit#heading=h.i48nii91fa3p)
3. set number of thread to 50 in "Thread Group" and run the test plan

Expected

Shows no error on server console.

What I actually see

SEVERE: Index: 0, Size: 0 when eval (Input!$J$15,Input!$J$21,Input!$J$22)
java.lang.IndexOutOfBoundsException: Index: 0, Size: 0

SEVERE: Index: 0, Size: 0 when eval (Input!$J$15,Input!$J$21,Input!$J$22)
java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.rangeCheck(Unknown Source)
at java.util.ArrayList.get(Unknown Source)
at org.zkoss.zss.model.impl.sys.formula.ParsingBook.getAnyExternalSheet(ParsingBook.java:183)
at org.zkoss.zss.model.impl.sys.formula.ParsingBook.getExternalSheet(ParsingBook.java:189)
at org.zkoss.zss.model.impl.sys.formula.EvalBook.getExternalSheet(EvalBook.java:134)
at org.zkoss.poi.ss.formula.OperationEvaluationContext.createExternSheetRefEvaluator(OperationEvaluationContext.java:81)
at org.zkoss.poi.ss.formula.OperationEvaluationContext.getRef3DEval(OperationEvaluationContext.java:266)
at org.zkoss.poi.ss.formula.WorkbookEvaluator.getEvalForPtg(WorkbookEvaluator.java:716)
at org.zkoss.poi.ss.formula.WorkbookEvaluator.evaluateFormula(WorkbookEvaluator.java:581)
at org.zkoss.poi.ss.formula.WorkbookEvaluator.evaluateAny(WorkbookEvaluator.java:941)
at org.zkoss.poi.ss.formula.WorkbookEvaluator.evaluate(WorkbookEvaluator.java:862)
at org.zkoss.zss.model.impl.sys.formula.FormulaEngineImpl.evaluateFormula(FormulaEngineImpl.java:389)
at org.zkoss.zss.model.impl.sys.formula.FormulaEngineImpl.evaluate(FormulaEngineImpl.java:349)
at org.zkoss.zss.model.impl.chart.SeriesImpl.evalFormula(SeriesImpl.java:78)
at org.zkoss.zss.model.impl.chart.SeriesImpl.getNumOfValue(SeriesImpl.java:125)
at org.zkoss.zssex.util.ChartHelper.preparePieModel(ChartHelper.java:379)
at org.zkoss.zssex.util.ChartHelper.drawChart(ChartHelper.java:142)
at org.zkoss.zssex.ui.widget.ChartWidget.initChart(ChartWidget.java:121)
at org.zkoss.zssex.ui.widget.ChartWidget.<init>(ChartWidget.java:46)
at org.zkoss.zssex.ui.widget.DefaultBookWidgetLoader.newChartWidget(DefaultBookWidgetLoader.java:411)
at org.zkoss.zssex.ui.widget.DefaultBookWidgetLoader.newChartWidgetGroup(DefaultBookWidgetLoader.java:404)
at org.zkoss.zssex.ui.widget.DefaultBookWidgetLoader.prepareChartWidgets(DefaultBookWidgetLoader.java:172)
at org.zkoss.zssex.ui.widget.DefaultBookWidgetLoader.onSheetSelected(DefaultBookWidgetLoader.java:82)
at org.zkoss.zss.ui.Spreadsheet.afterSheetSelected(Spreadsheet.java:4269)
at org.zkoss.zss.ui.Spreadsheet.getSelectedSSheet(Spreadsheet.java:809)
at org.zkoss.zss.ui.Spreadsheet.doInvalidate(Spreadsheet.java:4400)
at org.zkoss.zss.ui.Spreadsheet.invalidate(Spreadsheet.java:3774)
at org.zkoss.zss.ui.Spreadsheet.setSBook(Spreadsheet.java:634)
at org.zkoss.zss.ui.Spreadsheet.setBook(Spreadsheet.java:5100)
at org.zkoss.zss.benchmark.SharedBookComposer.doAfterCompose(SharedBookComposer.java:26)
at org.zkoss.zk.ui.impl.UiEngineImpl.doAfterCompose(UiEngineImpl.java:578)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild0(UiEngineImpl.java:880)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:826)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:735)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:797)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:757)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate(UiEngineImpl.java:699)
at org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage0(UiEngineImpl.java:442)
at org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage(UiEngineImpl.java:356)
at org.zkoss.zk.ui.http.DHtmlLayoutServlet.process(DHtmlLayoutServlet.java:215)
at org.zkoss.zk.ui.http.DHtmlLayoutServlet.doGet(DHtmlLayoutServlet.java:136)
...