diff --git a/src/GraphQL/Helper.php b/src/GraphQL/Helper.php
index 685b7be8..10a313fb 100644
--- a/src/GraphQL/Helper.php
+++ b/src/GraphQL/Helper.php
@@ -131,7 +131,7 @@ public static function buildSqlCondition($defaultTable, $q, $op = null, $subject
                                     $valuePart = ' =' . $db->quote($objectValue) . ')';
                                 }
 
-                                if (isset($fieldMappingTable, $key)) {
+                                if (isset($fieldMappingTable[$key])) {
                                     $parts[] = '( NOT ' . $db->quoteIdentifier($key) . $valuePart . ')';
                                 } else {
                                     $parts[] = '( NOT ' . self::quoteAbsoluteColumnName($defaultTable, $key) . $valuePart . ')';