diff --git a/src/Helpers/FixtureHelper.php b/src/Helpers/FixtureHelper.php
index 22c7620f..19feaeb7 100644
--- a/src/Helpers/FixtureHelper.php
+++ b/src/Helpers/FixtureHelper.php
@@ -24,7 +24,7 @@ public static function recursivelyReplaceAttributes(array $source, array $rules,
 
         array_walk_recursive($source, static function (&$value, $key) use ($rules, $caseSensitiveKeys) {
             if ($caseSensitiveKeys === false) {
-                $key = mb_strtolower($key);
+                $key = mb_strtolower((string) $key);
             }
 
             if (! array_key_exists($key, $rules)) {