diff --git a/src/RowModifier/ExternalCategoryManagement.php b/src/RowModifier/ExternalCategoryManagement.php
index 6a039a3..e3452bd 100644
--- a/src/RowModifier/ExternalCategoryManagement.php
+++ b/src/RowModifier/ExternalCategoryManagement.php
@@ -231,6 +231,6 @@ private function extractCategoriesFromString($categories)
     {
         return array_filter(array_map(function ($category) {
             return trim($category, " \t\n\r\0\x0B/");
-        }, explode(',', trim($categories, " \t\n\r\0\x0B/"))));
+        }, explode(',', trim($categories ?? '', " \t\n\r\0\x0B/"))));
     }
 }