Fixed a bug in which NULL values were not being preserved
diff --git a/system/libraries/Model.php b/system/libraries/Model.php
index 3b5123f..1d3cbdf 100644
--- a/system/libraries/Model.php
+++ b/system/libraries/Model.php
@@ -65,8 +65,7 @@
 				// problems so we'll conditionally use them

 				if ($use_reference == TRUE)

 				{

-					// Needed to prevent reference errors with some configurations

-					$this->$key = '';

+					$this->$key = NULL; // Needed to prevent reference errors with some configurations

 					$this->$key =& $CI->$key;

 				}

 				else