diff --git a/classes/ezfindservercallfunctions.php b/classes/ezfindservercallfunctions.php
index <HASH>..<HASH> 100644
--- a/classes/ezfindservercallfunctions.php
+++ b/classes/ezfindservercallfunctions.php
@@ -175,7 +175,7 @@ class eZFindServerCallFunctions
                 $classQueryParts = array();
                 foreach( $classes as $class )
                 {
-                    if ( is_string( $class ) )
+                    if ( !is_numeric( $class ) )
                     {
                         if ( $class = eZContentClass::fetchByIdentifier( $class ) )
                         {
Fix EZP-<I>: Support content class IDs as filters