Steps to reproduce

1. load the attached file

actual result

INFO: The specified formula '' contains an error. Expects cell reference or constant literal. when parsing at [t2262.xls]Sheet1!null
org.zkoss.poi.ss.formula.FormulaParseException: The specified formula '' contains an error. Expects cell reference or constant literal.
at org.zkoss.poi.ss.formula.FormulaParser.expected(FormulaParser.java:1978)
at org.zkoss.poi.ss.formula.FormulaParser.parseSimpleFactor(FormulaParser.java:1512)
at org.zkoss.poi.ss.formula.FormulaParser.percentFactor(FormulaParser.java:1467)
at org.zkoss.poi.ss.formula.FormulaParser.powerFactor(FormulaParser.java:1454)
at org.zkoss.poi.ss.formula.FormulaParser.Term(FormulaParser.java:1809)
at org.zkoss.poi.ss.formula.FormulaParser.additiveExpression(FormulaParser.java:1909)
at org.zkoss.poi.ss.formula.FormulaParser.concatExpression(FormulaParser.java:1893)
at org.zkoss.poi.ss.formula.FormulaParser.comparisonExpression(FormulaParser.java:1850)
at org.zkoss.poi.ss.formula.FormulaParser.unionExpression(FormulaParser.java:1830)
at org.zkoss.poi.ss.formula.FormulaParser.parse(FormulaParser.java:1951)
at org.zkoss.poi.ss.formula.FormulaParser.parse(FormulaParser.java:299)
at org.zkoss.zssex.formula.FormulaEngineEx.parse(FormulaEngineEx.java:81)
at org.zkoss.zss.model.impl.sys.formula.FormulaEngineImpl.parse0(FormulaEngineImpl.java:204)
at org.zkoss.zss.model.impl.sys.formula.FormulaEngineImpl.parse(FormulaEngineImpl.java:177)
at org.zkoss.zss.model.impl.chart.GeneralChartDataImpl.setCategoriesFormula(GeneralChartDataImpl.java:140)
at org.zkoss.zss.range.impl.imexp.ExcelXlsImporter.importSeries(ExcelXlsImporter.java:287)
at org.zkoss.zss.range.impl.imexp.ExcelXlsImporter.importChart(ExcelXlsImporter.java:114)
at org.zkoss.zss.range.impl.imexp.ExcelXlsImporter.importDrawings(ExcelXlsImporter.java:220)
at org.zkoss.zss.range.impl.imexp.AbstractExcelImporter.imports(AbstractExcelImporter.java:162)
at org.zkoss.zss.range.impl.imexp.ExcelImportAdapter.imports(ExcelImportAdapter.java:46)
at org.zkoss.zss.range.impl.imexp.AbstractImporter.imports(AbstractImporter.java:54)
at org.zkoss.zss.ui.Spreadsheet.getSBook(Spreadsheet.java:647)
at org.zkoss.zss.ui.Spreadsheet.getSelectedSSheet(Spreadsheet.java:845)
at org.zkoss.zss.ui.Spreadsheet.doInvalidate(Spreadsheet.java:4668)
at org.zkoss.zss.ui.Spreadsheet.invalidate(Spreadsheet.java:4007)
at org.zkoss.zss.ui.Spreadsheet.setSrc(Spreadsheet.java:885)

debug info

• zss still can display the chart