Index: src/ooxml/java/org/apache/poi/xssf/usermodel/helpers/XSSFFormulaUtils.java
===================================================================
--- src/ooxml/java/org/apache/poi/xssf/usermodel/helpers/XSSFFormulaUtils.java	(revisione 217363)
+++ src/ooxml/java/org/apache/poi/xssf/usermodel/helpers/XSSFFormulaUtils.java	(copia locale)
@@ -125,8 +125,7 @@
         if (ptg instanceof Pxg) {
             Pxg pxg = (Pxg)ptg;
             if (pxg.getExternalWorkbookNumber() < 1) {
-            	if (pxg.getSheetName().equals(oldName)) {			 
+               String sheetName = pxg.getSheetName();
+				if (sheetName != null && sheetName.equals(oldName)) {
                     pxg.setSheetName(newName);
                 }
             }