Index: wp-includes/meta.php
===================================================================
--- wp-includes/meta.php	(revision 23179)
+++ wp-includes/meta.php	(working copy)
@@ -717,7 +717,7 @@
 		// Split out the meta_key only queries (we can only do this for OR)
 		if ( 'OR' == $this->relation ) {
 			foreach ( $this->queries as $k => $q ) {
-				if ( ! isset( $q['value'] ) && ! empty( $q['key'] ) )
+				if ( ! array_key_exists( 'value', $q ) && ! empty( $q['key'] ) )
 					$key_only_queries[$k] = $q;
 				else
 					$queries[$k] = $q;
@@ -743,6 +743,9 @@
 			elseif ( ! in_array( $meta_type, array( 'BINARY', 'CHAR', 'DATE', 'DATETIME', 'DECIMAL', 'SIGNED', 'TIME', 'UNSIGNED' ) ) )
 				$meta_type = 'CHAR';
 
+			if ( array_key_exists( 'value', $q ) && is_null( $q['value'] ) )
+				$q['value'] = '';
+
 			$meta_value = isset( $q['value'] ) ? $q['value'] : null;
 
 			if ( isset( $q['compare'] ) )