Created attachment 30718 [details] java main() putting formula before sheet create 1) Create a workbook. 2) Add sheet "X" 3) Add cell formula at 0,0 "=2+Y!A1" --> getCellFormula() has lost "Y" sheet name 4) Evaluate cell's formula --> java.lang.IllegalArgumentException: Invalid sheetIndex: -1. I'm not sure if this is by design, but it can make it difficult to write a library that outputs formulas and sheets in a convenient order. (sometimes sheet may not exist, but it's a good time to add the formula anyways. Excel itself shows #VALUE on screen but doesn't wipe out the cell's formula which may be restored by clicking.)