Index: lams_gradebook/src/java/org/lamsfoundation/lams/gradebook/service/GradebookService.java
===================================================================
diff -u -r52dd63e24e52ed79e408faafa56a4677885f76d1 -rf59c54692eae777e0f10c71f59c1f104280a320b
--- lams_gradebook/src/java/org/lamsfoundation/lams/gradebook/service/GradebookService.java	(.../GradebookService.java)	(revision 52dd63e24e52ed79e408faafa56a4677885f76d1)
+++ lams_gradebook/src/java/org/lamsfoundation/lams/gradebook/service/GradebookService.java	(.../GradebookService.java)	(revision f59c54692eae777e0f10c71f59c1f104280a320b)
@@ -232,7 +232,8 @@
     }
 
     @Override
-    public List<GradebookGridRowDTO> getGBActivityRowsForLesson(Long lessonId, TimeZone userTimezone, boolean escapeTitles) {
+    public List<GradebookGridRowDTO> getGBActivityRowsForLesson(Long lessonId, TimeZone userTimezone,
+	    boolean escapeTitles) {
 	GradebookService.logger.debug("Getting gradebook data for lesson: " + lessonId);
 
 	Lesson lesson = lessonService.getLesson(lessonId);
@@ -1744,9 +1745,9 @@
 	    ToolActivity activity = toolSession.getToolActivity();
 	    GradebookUserActivity gradebookUserActivity = getGradebookUserActivity(activity.getActivityId(), userID);
 	    if (gradebookUserActivity != null) {
-		// act as if the mark was updated to 0 so lesson mark calculates correctly
+		// set mark to null so lesson mark recalculates correctly
 		Double oldActivityMark = gradebookUserActivity.getMark();
-		gradebookUserActivity.setMark(0D);
+		gradebookUserActivity.setMark(null);
 		Lesson lesson = (Lesson) activity.getLearningDesign().getLessons().iterator().next();
 		boolean isWeightedMarks = toolService.isWeightedMarks(lesson.getLearningDesign());
 		GradebookUserLesson gradebookUserLesson = getGradebookUserLesson(lesson.getLessonId(), userID);
@@ -1917,11 +1918,16 @@
      */
     private void aggregateTotalMarkForLesson(boolean useWeightings, GradebookUserLesson gradebookUserLesson,
 	    GradebookUserActivity markedActivity, Double oldActivityMark) {
-
 	List<GradebookUserActivity> userActivities = gradebookDAO.getGradebookUserActivitiesForLesson(
 		gradebookUserLesson.getLesson().getLessonId(), gradebookUserLesson.getLearner().getUserId());
+	// if there is only one marked activity and it is being deleted, remove whole lesson mark
+	if (markedActivity.getMark() == null && userActivities.size() == 1
+		&& userActivities.get(0).getUid() == markedActivity.getUid()) {
+	    gradebookDAO.delete(gradebookUserLesson);
+	    return;
+	}
 
-	Double totalMark;
+	Double totalMark = null;
 	if (oldActivityMark == null || gradebookUserLesson.getMark() == null) {
 	    totalMark = calculateLessonMark(useWeightings, userActivities, markedActivity);
 	} else if (!useWeightings) {
@@ -1931,7 +1937,8 @@
 	    }
 	} else {
 	    Double oldWeightedMark = getWeightedMark(true, markedActivity, oldActivityMark);
-	    Double newWeighterMark = getWeightedMark(true, markedActivity, markedActivity.getMark());
+	    Double newWeighterMark = markedActivity.getMark() == null ? 0
+		    : getWeightedMark(true, markedActivity, markedActivity.getMark());
 	    totalMark = gradebookUserLesson.getMark() - oldWeightedMark + newWeighterMark;
 	}