diff --git a/src/main/java/org/gcube/portlets/user/td/unionwizardwidget/client/ColumnMappingPanel.java b/src/main/java/org/gcube/portlets/user/td/unionwizardwidget/client/ColumnMappingPanel.java
index c59e97c..dd74566 100644
--- a/src/main/java/org/gcube/portlets/user/td/unionwizardwidget/client/ColumnMappingPanel.java
+++ b/src/main/java/org/gcube/portlets/user/td/unionwizardwidget/client/ColumnMappingPanel.java
@@ -455,11 +455,13 @@ public class ColumnMappingPanel extends ContentPanel {
 					@Override
 					public void onSelection(SelectionEvent<ColumnData> event) {
 						comboUnionColumn.reset();
+						comboUnionColumn.clear();
 						storeComboUnionColumn.clear();
-
+						storeComboUnionColumn.commitChanges();
+						
 						ColumnData selectedSourceColumn = event
 								.getSelectedItem();
-						storeComboUnionColumn.clear();
+	
 						for (ColumnData col : unionColumns) {
 							if (selectedSourceColumn.getDataTypeName()
 									.compareTo(ColumnDataType.Text.toString()) == 0