--- a/bundles/org.eclipse.e4.ui.model.workbench/src/org/eclipse/e4/ui/model/internal/ModelUtils.java	
+++ a/bundles/org.eclipse.e4.ui.model.workbench/src/org/eclipse/e4/ui/model/internal/ModelUtils.java	
@@ -131,7 +131,9 @@ 
 						break;
 					}
 				} else {
-					System.err.println("Not a valid list position.");
+					System.err.println("Position '" + positionInList + "' defined in '" 
+							+ container.getElementId() + "' is no not a valid list position. See"
+							+ Position.class.getCanonicalName() + " for valid definitions.");
 				}