Index: branches/5.2.x/core/kernel/utility/validator.php
===================================================================
--- branches/5.2.x/core/kernel/utility/validator.php
+++ branches/5.2.x/core/kernel/utility/validator.php
@@ -264,13 +264,15 @@
 			return false;
 		}
 
-		// validate string
-		if ( isset($params['max_len']) ) {
-			$res = $res && mb_strlen($val) <= $params['max_len'];
-		}
+		if ( strlen($val) > 0 ) {
+			// Validate string.
+			if ( isset($params['max_len']) ) {
+				$res = $res && mb_strlen($val) <= $params['max_len'];
+			}
 
-		if ( isset($params['min_len']) ) {
-			$res = $res && mb_strlen($val) >= $params['min_len'];
+			if ( isset($params['min_len']) ) {
+				$res = $res && mb_strlen($val) >= $params['min_len'];
+			}
 		}
 
 		if ( !$res ) {