Index: lams_gradebook/src/java/org/lamsfoundation/lams/gradebook/service/GradebookService.java
===================================================================
diff -u -r1ad00988450cb1238e0f36f7f398ae3c45328951 -rab88b7ff6a0dbd05c91a29e135b11f9596674f4c
--- lams_gradebook/src/java/org/lamsfoundation/lams/gradebook/service/GradebookService.java	(.../GradebookService.java)	(revision 1ad00988450cb1238e0f36f7f398ae3c45328951)
+++ lams_gradebook/src/java/org/lamsfoundation/lams/gradebook/service/GradebookService.java	(.../GradebookService.java)	(revision ab88b7ff6a0dbd05c91a29e135b11f9596674f4c)
@@ -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);
 	}
@@ -1193,10 +1193,11 @@
 	    userDataRow.addCell(userRow.getLogin());
 	    userDataRow.addCell(getProgressMessage(userRow));
 	    userDataRow.addCell(userRow.getTimeTakenSeconds());
+	    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());
+		userDataRow.addCell(userMark);
 	    }
 	}
 	summarySheet.addEmptyRow();
@@ -1254,8 +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());
 	    }
@@ -1568,7 +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 -r670dd8e248dc4705c92031c79d05a99559e35024 -rab88b7ff6a0dbd05c91a29e135b11f9596674f4c
--- lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/service/AssessmentServiceImpl.java	(.../AssessmentServiceImpl.java)	(revision 670dd8e248dc4705c92031c79d05a99559e35024)
+++ lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/service/AssessmentServiceImpl.java	(.../AssessmentServiceImpl.java)	(revision ab88b7ff6a0dbd05c91a29e135b11f9596674f4c)
@@ -2173,18 +2173,21 @@
 		|| question.getType() == QbQuestion.TYPE_VERY_SHORT_ANSWERS
 		|| question.getType() == QbQuestion.TYPE_NUMERICAL) {
 	    for (QbOption option : question.getQbQuestion().getQbOptions()) {
-		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.getMaxMark() > 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.getQbQuestion().getCorrectAnswer()) {
@@ -2193,7 +2196,8 @@
 		wrongCell.setColor(IndexedColors.GREEN);
 	    }
 	}
-	Double summaryNAPercentage = (double) summaryNACount / total;
+
+	Double summaryNAPercentage = total == 0 ? 0 : (double) summaryNACount / total;
 	summaryTableRow.addPercentageCell(summaryNAPercentage);
 
 	return summaryTableRow;