Index: lams_central/src/java/org/lamsfoundation/lams/authoring/ObjectExtractor.java
===================================================================
diff -u -r64e6623bc68b11257c7f5d71f3953154f80b6fde -r2d51ee8a959ecba6a2f6ac2da80b7b9857bc50d0
--- lams_central/src/java/org/lamsfoundation/lams/authoring/ObjectExtractor.java	(.../ObjectExtractor.java)	(revision 64e6623bc68b11257c7f5d71f3953154f80b6fde)
+++ lams_central/src/java/org/lamsfoundation/lams/authoring/ObjectExtractor.java	(.../ObjectExtractor.java)	(revision 2d51ee8a959ecba6a2f6ac2da80b7b9857bc50d0)
@@ -780,7 +780,9 @@
 	    evaluation.setToolOutputDefinition(toolOutputDefinition);
 
 	    String weight = (String) JsonUtil.opt(activityDetails, AuthoringJsonTags.TOOL_OUTPUT_WEIGHT);
-	    if (StringUtils.isNotBlank(weight)) {
+	    if (StringUtils.isBlank(weight)) {
+		evaluation.setWeight(null);
+	    } else {
 		evaluation.setWeight(Integer.valueOf(weight));
 	    }
 	    toolActivity.setEvaluation(evaluation);
Index: lams_central/web/includes/javascript/authoring/authoringGeneral.js
===================================================================
diff -u -ra83cc751c307aa62dd4c7fa568dccfb7c51bdc60 -r2d51ee8a959ecba6a2f6ac2da80b7b9857bc50d0
--- lams_central/web/includes/javascript/authoring/authoringGeneral.js	(.../authoringGeneral.js)	(revision a83cc751c307aa62dd4c7fa568dccfb7c51bdc60)
+++ lams_central/web/includes/javascript/authoring/authoringGeneral.js	(.../authoringGeneral.js)	(revision 2d51ee8a959ecba6a2f6ac2da80b7b9857bc50d0)
@@ -1039,7 +1039,11 @@
 								'min'    : 0,
 								'max'    : 100,
 								'change' : function(){
-									activity.gradebookToolOutputWeight = $(this).val();
+									var value = $(this).val();
+									if (value == "" || isNaN(value)) {
+										value = null;
+									}
+									activity.gradebookToolOutputWeight = value;
 									layout.weightsDialog.data('sumWeights')();
 								},
 								'spin'  : function(event, ui) {