Index: lams_common/src/java/org/lamsfoundation/lams/util/excel/ExcelRow.java
===================================================================
diff -u -rf13b8a48b9a1cfaee56a654ba751a9736616b726 -r746369b4989a3ccae7c71d6a07bbb79ab4f47689
--- lams_common/src/java/org/lamsfoundation/lams/util/excel/ExcelRow.java	(.../ExcelRow.java)	(revision f13b8a48b9a1cfaee56a654ba751a9736616b726)
+++ lams_common/src/java/org/lamsfoundation/lams/util/excel/ExcelRow.java	(.../ExcelRow.java)	(revision 746369b4989a3ccae7c71d6a07bbb79ab4f47689)
@@ -30,11 +30,11 @@
 	return cell;
     }
     
-    public ExcelCell addPercentageCell(Object cellValue) {
+    public ExcelCell addPercentageCell(Double cellValue) {
 	return addPercentageCell(cellValue, false, 0);
     }
     
-    public ExcelCell addPercentageCell(Object cellValue, Boolean isBold, int borderStyle) {
+    public ExcelCell addPercentageCell(Double cellValue, Boolean isBold, int borderStyle) {
 	ExcelCell cell = new ExcelCell(cellValue);
 	cell.setDataFormat(ExcelCell.CELL_FORMAT_PERCENTAGE);
 	cells.add(cell);
Index: lams_gradebook/src/java/org/lamsfoundation/lams/gradebook/service/GradebookService.java
===================================================================
diff -u -r9c45f8415c6c06df681a4e94205aa71d123b0bfa -r746369b4989a3ccae7c71d6a07bbb79ab4f47689
--- lams_gradebook/src/java/org/lamsfoundation/lams/gradebook/service/GradebookService.java	(.../GradebookService.java)	(revision 9c45f8415c6c06df681a4e94205aa71d123b0bfa)
+++ lams_gradebook/src/java/org/lamsfoundation/lams/gradebook/service/GradebookService.java	(.../GradebookService.java)	(revision 746369b4989a3ccae7c71d6a07bbb79ab4f47689)
@@ -1131,7 +1131,7 @@
 	ExcelRow lessonAverageMark = summarySheet.initRow();
 	lessonAverageMark.addCell(getMessage("gradebook.export.average.lesson.mark"), true);
 	if (isWeighted) {
-	    lessonAverageMark.addPercentageCell(lessonAverageMarkValue / 100.0);
+	    lessonAverageMark.addPercentageCell(lessonAverageMarkValue == null ? null : lessonAverageMarkValue / 100.0);
 	} else {
 	    lessonAverageMark.addCell(lessonAverageMarkValue);
 	}
@@ -1193,11 +1193,11 @@
 	    userDataRow.addCell(userRow.getLogin());
 	    userDataRow.addCell(getProgressMessage(userRow));
 	    userDataRow.addCell(userRow.getTimeTakenSeconds());
-	    Double mark = userRow.getMark();
+	    Double userMark = userRow.getMark();
 	    if (isWeighted) {
-		userDataRow.addPercentageCell(mark == null ? 0 : mark / 100.0);
+		userDataRow.addPercentageCell(userMark == null ? null : userMark / 100.0);
 	    } else {
-		userDataRow.addCell(mark);
+		userDataRow.addCell(userMark);
 	    }
 	}
 	summarySheet.addEmptyRow();
@@ -1255,8 +1255,9 @@
 		}
 		userDataRow.addCell(userActivityMark);
 	    }
+	    Double userMark = userRow.getMark();
 	    if (isWeighted) {
-		userDataRow.addPercentageCell(userRow.getMark() / 100.0);
+		userDataRow.addPercentageCell(userMark == null ? null : userMark / 100.0);
 	    } else {
 		userDataRow.addCell(userRow.getMark());
 	    }
@@ -1569,7 +1570,7 @@
 		    userDataRow.addCell(timeTakenSeconds);
 		    userDataRow.addCell(feedback);
 		    if (isWeightedLessonMap.get(lesson.getLessonId())) {
-			userDataRow.addPercentageCell(mark / 100.0);
+			userDataRow.addPercentageCell(mark == null ? null : mark / 100.0);
 		    } else {
 			userDataRow.addCell(mark);
 		    }
Index: lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/service/AssessmentServiceImpl.java
===================================================================
diff -u -r20aa6cbca9fc96d341080e6ad39f82593443f792 -r746369b4989a3ccae7c71d6a07bbb79ab4f47689
--- lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/service/AssessmentServiceImpl.java	(.../AssessmentServiceImpl.java)	(revision 20aa6cbca9fc96d341080e6ad39f82593443f792)
+++ lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/service/AssessmentServiceImpl.java	(.../AssessmentServiceImpl.java)	(revision 746369b4989a3ccae7c71d6a07bbb79ab4f47689)
@@ -2011,18 +2011,21 @@
 		|| question.getType() == AssessmentConstants.QUESTION_TYPE_SHORT_ANSWER
 		|| question.getType() == AssessmentConstants.QUESTION_TYPE_NUMERICAL) {
 	    for (AssessmentQuestionOption option : question.getOptions()) {
-		Double optionPercentage = (double) summaryOfAnswers.get(option.getUid()) / total;
+		Double optionPercentage = total == 0 || summaryOfAnswers.get(option.getUid()) == null ? 0
+			: (double) summaryOfAnswers.get(option.getUid()) / total;
 		ExcelCell optionCell = summaryTableRow.addPercentageCell(optionPercentage);
 		if (option.getGrade() > 0) {
 		    optionCell.setColor(IndexedColors.GREEN);
 		}
 	    }
 	    
 	} else {
-	    Double correctPercentage = (double) summaryOfAnswers.get(trueKey) / total;
+	    Double correctPercentage = total == 0 || summaryOfAnswers.get(trueKey) == null ? 0
+		    : (double) summaryOfAnswers.get(trueKey) / total;
 	    ExcelCell correctCell = summaryTableRow.addPercentageCell(correctPercentage);
 	    
-	    Double wrongPercentage = (double) summaryOfAnswers.get(falseKey) / total;
+	    Double wrongPercentage = total == 0 || summaryOfAnswers.get(falseKey) == null ? 0
+		    : (double) summaryOfAnswers.get(falseKey) / total;
 	    ExcelCell wrongCell = summaryTableRow.addPercentageCell(wrongPercentage);
 	    
 	    if (question.getCorrectAnswer()) {
@@ -2032,7 +2035,7 @@
 	    }
 	}
 
-	Double summaryNAPercentage = (double) summaryNACount / total;
+	Double summaryNAPercentage = total == 0 ? 0 : (double) summaryNACount / total;
 	summaryTableRow.addPercentageCell(summaryNAPercentage);
 	    
 	return summaryTableRow;