diff --git a/src/View/Widget/InputGroupTrait.php b/src/View/Widget/InputGroupTrait.php
index dff49e01..027b4000 100644
--- a/src/View/Widget/InputGroupTrait.php
+++ b/src/View/Widget/InputGroupTrait.php
@@ -73,6 +73,7 @@ protected function _withInputGroup(array $data, ContextInterface $context): stri
                 $errorClass &&
                 $context->hasError($data['fieldName'])
             ) {
+                $attrs['class'] = (array)$attrs['class'];
                 $attrs['class'][] = $errorClass;
             }