diff --git a/src/Forms/Validator.php b/src/Forms/Validator.php
index fbf2700de..e970ffd6f 100644
--- a/src/Forms/Validator.php
+++ b/src/Forms/Validator.php
@@ -78,7 +78,7 @@ public static function formatMessage(Rule $rule, bool $withValue = true)
 						$caption = $caption instanceof Nette\Utils\IHtmlString
 							? $caption->getText()
 							: ($translator ? $translator->translate($caption) : $caption);
-						return rtrim($caption, ':');
+						return rtrim((string) $caption, ':');
 					}
 					return '';
 				case 'value': return $withValue ? $rule->control->getValue() : $m[0];