Attempt to modify a referenced format while setting background color

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\[code\]WritableFont headerFont = new WritableFont(WritableFont.ARIAL, 9, WritableFont.BOLD, false)WritableCellFormat headerFormat = new WritableCellFormat(headerFont)headerFormat.setBackground(Colour.GREY_25_PERCENT)headerFormat.setWrap(true)\[/code\]I am getting an Exception: jxl.write.biff.JxlWriteException: Attempt to modify a referenced format Why?