diff --git a/src/services/models/Callback.ts b/src/services/models/Callback.ts
index 956537afb2..05499f5afc 100644
--- a/src/services/models/Callback.ts
+++ b/src/services/models/Callback.ts
@@ -8,7 +8,8 @@ import { RedocNormalizedOptions } from '../RedocNormalizedOptions';
 
 export class CallbackModel {
   @observable
-  expanded: boolean;
+  expanded: boolean = false;
+
   name: string;
   operations: OperationModel[] = [];
 
diff --git a/src/services/models/Field.ts b/src/services/models/Field.ts
index d156b03958..ab30d0944f 100644
--- a/src/services/models/Field.ts
+++ b/src/services/models/Field.ts
@@ -39,7 +39,7 @@ const DEFAULT_SERIALIZATION: Record<
  */
 export class FieldModel {
   @observable
-  expanded: boolean | undefined;
+  expanded: boolean | undefined = false;
 
   schema: SchemaModel;
   name: string;
diff --git a/src/services/models/Response.ts b/src/services/models/Response.ts
index 2b4631cab7..f50ee0f1ca 100644
--- a/src/services/models/Response.ts
+++ b/src/services/models/Response.ts
@@ -10,7 +10,7 @@ import { MediaContentModel } from './MediaContent';
 
 export class ResponseModel {
   @observable
-  expanded: boolean;
+  expanded: boolean = false;
 
   content?: MediaContentModel;
   code: string;