Index: lams_central/src/java/org/lamsfoundation/lams/webservice/xml/LessonManagerServlet.java
===================================================================
diff -u -r878e6947f0f2e48ca892db589719d73064dcf034 -r604c9ed9dd3e2668157446b892889d873299201c
--- lams_central/src/java/org/lamsfoundation/lams/webservice/xml/LessonManagerServlet.java	(.../LessonManagerServlet.java)	(revision 878e6947f0f2e48ca892db589719d73064dcf034)
+++ lams_central/src/java/org/lamsfoundation/lams/webservice/xml/LessonManagerServlet.java	(.../LessonManagerServlet.java)	(revision 604c9ed9dd3e2668157446b892889d873299201c)
@@ -138,6 +138,9 @@
 			Document document = builder.newDocument();
 
 			Element element = null;
+			
+			if (hashValue==null || hashValue.equals(""))
+				throw new NullPointerException("Hash value missing in parameters");
 
 			if (method.equals(CentralConstants.METHOD_START)) {
 				ldId = new Long(ldIdStr);
@@ -221,7 +224,11 @@
 		} catch (ParserConfigurationException e) {
 			log.error("Can not build XML document", e);
 			response.sendError(response.SC_INTERNAL_SERVER_ERROR);
-		} catch (Exception e) {
+		} catch (NullPointerException e) {
+			log.error("Missing parameters", e);
+			response.sendError(response.SC_INTERNAL_SERVER_ERROR);
+		}
+		catch (Exception e) {
 			log.error("Problem loading learning manager servlet request", e);
 			response.sendError(response.SC_INTERNAL_SERVER_ERROR);
 		}