diff --git a/packages/volto/src/components/manage/Widgets/SchemaWidget.jsx b/packages/volto/src/components/manage/Widgets/SchemaWidget.jsx
index 858266ae8b..ad4d5ca5eb 100644
--- a/packages/volto/src/components/manage/Widgets/SchemaWidget.jsx
+++ b/packages/volto/src/components/manage/Widgets/SchemaWidget.jsx
@@ -1741,7 +1741,7 @@ class SchemaWidget extends Component {
           <ModalForm
             onSubmit={this.onAddField}
             onCancel={this.onCancel}
-            className={`field-${slugify(this.state.addField || 'label_text_field')}`}
+            className={`field-${slugify(isString(this.state.addField) && this.state.addField !== '' ? this.state.addField : 'label_text_field')}`}
             onChangeFormData={(data) => {
               this.setState({
                 addField: data.factory,