Index: classes/ezfezpsolrquerybuilder.php
===================================================================
--- classes/ezfezpsolrquerybuilder.php  (revision 730)
+++ classes/ezfezpsolrquerybuilder.php  (working copy)
@@ -1446,7 +1446,7 @@
             // Fetch class list.
             $condArray = array( "is_searchable" => 1,
                                 "version" => eZContentClass::VERSION_STATUS_DEFINED );
-            if ( !is_array( $classIDArray ) )
+            if ( !is_array( $classIDArray ) && !empty( $classIDArray ) )
             {
                 $classIDArray = array( $classIDArray );
             }