Index: lams_central/web/includes/javascript/authoring/authoringGeneral.js
===================================================================
RCS file: /usr/local/cvsroot/lams_central/web/includes/javascript/authoring/authoringGeneral.js,v
diff -u -r1.51.2.60 -r1.51.2.61
--- lams_central/web/includes/javascript/authoring/authoringGeneral.js	23 Dec 2016 01:23:35 -0000	1.51.2.60
+++ lams_central/web/includes/javascript/authoring/authoringGeneral.js	29 Dec 2016 09:59:12 -0000	1.51.2.61
@@ -2574,7 +2574,7 @@
 		}
 		layout.modified = modified;
 		if (!modified && layout.activities.length > 0) {
-			$('#previewButton').prop('disabled', layout.ld.invalid);
+			$('#previewButton').prop('disabled', layout.ld.invalid === true);
 			$('#exportLamsButton').removeClass('disabled');
 			$('#ldDescriptionFieldModified').text('');
 		} else {