diff --git a/system/Validation/Validation.php b/system/Validation/Validation.php
index 0b49fe02c48b..b10711978068 100644
--- a/system/Validation/Validation.php
+++ b/system/Validation/Validation.php
@@ -337,7 +337,7 @@ protected function processRules(
 
                 // @phpstan-ignore-next-line $error may be set by rule methods.
                 $this->errors[$field] = $error ?? $this->getErrorMessage(
-                    ($this->isClosure($rule) || $arrayCallable) ? $i : $rule,
+                    ($this->isClosure($rule) || $arrayCallable) ? (string) $i : $rule,
                     $field,
                     $label,
                     $param,