 		 */
 		public function deriveStringFromNumber(index:Number):String
 		{
-			// validate lookup table if necessary
-			if (_keyToIndexMap == null)
-				createLookupTable();
-			// return the key at the given number as an index
+			if (_triggerCount != triggerCounter)
+			{
+				_triggerCount = triggerCounter;
+				validate();
+			}
+			
 			index = Math.round(index);
-			// return '' if there is no key at the given index value
-			if (index < 0 || index >= _keys.length)
+			if (!_column || index < 0 || index >= _keys.length)
 				return '';
-			return getInternalColumn() ? getInternalColumn().getValueFromKey(_keys[index], String) : '';
+			return _column.getValueFromKey(_keys[index], String);
 		}
 	}
 }

		 */
		override public function getValueFromKey(key:IQualifiedKey, dataType:Class = null):*
		{
			var result:*;
			if (getInternalColumn() != null)
			{
				// validate lookup table if necessary
				if (_keyToIndexMap == null)
					createLookupTable();
				// get the list of internal keys from the given stringValue
				result = Number(_keyToIndexMap[key]);
			}
			else
			{
				result = NaN;
			}
			// cast to other types
			if (dataType == String)
				result = getInternalColumn() ? getInternalColumn().getValueFromKey(key, String) : '';
			else if (dataType == Boolean)
				result = !isNaN(result); // true if key exists in lookup table
			
			return result;
		}
		
		/**

		 * @return The compare result used to sort a list of keys.
		 */
		private function sortByNumericValue(key1:IQualifiedKey, key2:IQualifiedKey):int
		{
			var val1:Number = _column.getValueFromKey(key1, Number);
			var val2:Number = _column.getValueFromKey(key2, Number);
			// if numeric values are equal, compare the keys
			return ObjectUtil.numericCompare(val1, val2)
				|| QKeyManager.keyCompare(key1, key2);
		}
		
		override public function get keys():Array
		{
			if (_triggerCount != triggerCounter)
			{
				_triggerCount = triggerCounter;
				validate();
			}
			return _keys;
		}
		
		/**
		 * @param key A key existing in the internal column.
