SEVERE: >>java.util.EmptyStackException 
>>        at java.util.Stack.peek(Unknown Source) 
>>        at java.util.Stack.pop(Unknown Source) 
>>        at org.zkoss.poi.ss.formula.WorkbookEvaluator.evaluateFormula(WorkbookEvaluator.java:561) 
>>        at org.zkoss.poi.ss.formula.WorkbookEvaluator.evaluateAny(WorkbookEvaluator.java:313) 
>>        at org.zkoss.poi.ss.formula.WorkbookEvaluator.evaluate(WorkbookEvaluator.java:252) 
>>        at org.zkoss.poi.hssf.usermodel.HSSFFormulaEvaluator.evaluateFormulaCellValue(HSSFFormulaEvaluator.java:357) 
>>        at org.zkoss.poi.hssf.usermodel.HSSFFormulaEvaluator.evaluate(HSSFFormulaEvaluator.java:188) 
>>        at org.zkoss.zss.model.impl.BookHelper.evaluate(BookHelper.java:471) 
>>        at org.zkoss.zss.model.impl.BookHelper.reevaluate(BookHelper.java:303) 
>>        at org.zkoss.zss.model.impl.BookHelper.reevaluateAndNotify(BookHelper.java:457) 
>>        at org.zkoss.zss.model.impl.RangeImpl.reevaluateAndNotify(RangeImpl.java:421) 
>>        at org.zkoss.zss.model.impl.RangeImpl.setEditText(RangeImpl.java:385) 
>>        at org.zkoss.zss.ui.impl.Utils.setEditText(Utils.java:914) 
>>        at org.zkoss.zss.ui.impl.Utils.setEditText(Utils.java:904) 
>>        at org.zkoss.zss.ui.impl.Utils.setEditTextWithValidation(Utils.java:1240) 
>>        at org.zkoss.zss.ui.Spreadsheet.processStopEditing0(Spreadsheet.java:4386) 
>>        at org.zkoss.zss.ui.Spreadsheet.processStopEditing(Spreadsheet.java:4371) 
>>        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
>>        at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) 
>>        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) 
>>        at java.lang.reflect.Method.invoke(Unknown Source) 
>>        at com.comerica.zk.COSheet$7.onEvent(COSheet.java:330) 
>>        at org.zkoss.zk.ui.AbstractComponent.onEvent(AbstractComponent.java:2743) 
>>        at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2714) 
>>        at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2655) 
>>        at com.comerica.zk.COSheet.service(COSheet.java:378) 
>>        at org.zkoss.zk.ui.impl.EventProcessor.process(EventProcessor.java:136) 
>>        at org.zkoss.zk.ui.impl.UiEngineImpl.processEvent(UiEngineImpl.java:1721) 
>>        at org.zkoss.zk.ui.impl.UiEngineImpl.process(UiEngineImpl.java:1506) 
>>        at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1216) 
>>        at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:601) 
>>        at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:483) 
>>        at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:492) 
>>        at javax.servlet.http.HttpServlet.service(HttpServlet.java:647) 
>>        at javax.servlet.http.HttpServlet.service(HttpServlet.java:728) 
>>        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305) 
>>        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210) 
>>        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222) 
>>        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123) 
>>        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502) 
>>        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171) 
>>        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99) 
>>        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953) 
>>        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118) 
>>        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408) 
>>        at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1023) 
>>        at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589) 
>>        at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310) 
>>        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) 
>>        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) 
>>        at java.lang.Thread.run(Unknown Source)