diff --git a/src/main/java/org/olat/ims/qti/statistics/ui/QTI12AssessmentStatisticsController.java b/src/main/java/org/olat/ims/qti/statistics/ui/QTI12AssessmentStatisticsController.java
index bc33e4d0fb5b106c028b81f1a66c938c52c768b1..482afdac97c1760b9ed2d8c079320b4339d4787d 100644
--- a/src/main/java/org/olat/ims/qti/statistics/ui/QTI12AssessmentStatisticsController.java
+++ b/src/main/java/org/olat/ims/qti/statistics/ui/QTI12AssessmentStatisticsController.java
@@ -261,14 +261,16 @@ public class QTI12AssessmentStatisticsController extends BasicController {
 		for(StatisticSurveyItem surveyItem:surveyItems) {
 			Item item = surveyItem.getItem();
 			Series series = seriesfactory.getSeries(item, null);
-			String name = "overview_" + count++;
-			VelocityContainer vc = createVelocityContainer(name, "hbar_item_overview");
-			vc.contextPut("series", series);
-			vc.contextPut("question", item.getQuestion().getQuestion().renderAsHtml(mediaBaseURL));
-			vc.contextPut("questionType", item.getQuestion().getType());
-			vc.contextPut("title", item.getTitle());
-			mainVC.put(vc.getDispatchID(), vc);
-			overviewList.add(vc.getDispatchID());
+			if(series != null) {//essay hasn't a series
+				String name = "overview_" + count++;
+				VelocityContainer vc = createVelocityContainer(name, "hbar_item_overview");
+				vc.contextPut("series", series);
+				vc.contextPut("question", item.getQuestion().getQuestion().renderAsHtml(mediaBaseURL));
+				vc.contextPut("questionType", item.getQuestion().getType());
+				vc.contextPut("title", item.getTitle());
+				mainVC.put(vc.getDispatchID(), vc);
+				overviewList.add(vc.getDispatchID());
+			}
 		}
 		
 		mainVC.contextPut("overviewList", overviewList);
diff --git a/src/main/resources/database/mysql/alter_8_4_0_to_9_0_0.sql b/src/main/resources/database/mysql/alter_8_4_0_to_9_0_0.sql
index 8b714042b67494dbc4845bc747f31266fa3139af..8f7dc6dda5ddbfcf8d85c07914d2f0a15ca1b6b5 100644
--- a/src/main/resources/database/mysql/alter_8_4_0_to_9_0_0.sql
+++ b/src/main/resources/database/mysql/alter_8_4_0_to_9_0_0.sql
@@ -330,6 +330,7 @@ alter table o_qp_item add constraint idx_qp_item_type_id foreign key (fk_type) r
 alter table o_qp_item add constraint idx_qp_item_license_id foreign key (fk_license) references o_qp_license(id);
 
 alter table o_qp_taxonomy_level add constraint idx_qp_field_2_parent_id foreign key (fk_parent_field) references o_qp_taxonomy_level(id);
+create index idx_taxon_mat_pathon on o_qp_taxonomy_level (q_mat_path_ids(255));
 
 alter table o_qp_item_type add unique (q_type(200));
 
diff --git a/src/main/resources/database/mysql/alter_9_3_0_to_9_4_0.sql b/src/main/resources/database/mysql/alter_9_3_0_to_9_4_0.sql
index 8171a9df4a2a4104cee51b0ba87557ad0332345b..58c2b3929b0e8744f7c945ddd92fd90f6d59961c 100644
--- a/src/main/resources/database/mysql/alter_9_3_0_to_9_4_0.sql
+++ b/src/main/resources/database/mysql/alter_9_3_0_to_9_4_0.sql
@@ -88,6 +88,8 @@ alter table o_cl_check add constraint check_box_ctx foreign key (fk_checkbox_id)
 alter table o_cl_check add unique check_identity_unique_ctx (fk_identity_id, fk_checkbox_id);
 create index idx_checkbox_uuid_idx on o_cl_checkbox (c_checkboxid);
 
+-- missing index
+create index idx_taxon_mat_pathon on o_qp_taxonomy_level (q_mat_path_ids(255));