diff --git a/src/FilterService/FilterType/MultiSelectCategory.php b/src/FilterService/FilterType/MultiSelectCategory.php
index 9e5982f9..c4d43eec 100644
--- a/src/FilterService/FilterType/MultiSelectCategory.php
+++ b/src/FilterService/FilterType/MultiSelectCategory.php
@@ -96,7 +96,7 @@ public function addCondition(AbstractFilterDefinitionType $filterDefinition, Pro
                     $category = $category->getId();
                 }
 
-                $category = '%,' . trim($category) . ',%';
+                $category = '%,' . trim((string)$category) . ',%';
 
                 $conditions[] = $filterDefinition->getField() . ' LIKE ' . $db->quote($category);
             }