diff --git a/core/field.ts b/core/field.ts
index ce34c8a87..cf9148e1c 100644
--- a/core/field.ts
+++ b/core/field.ts
@@ -45,17 +45,17 @@ import * as WidgetDiv from './widgetdiv.js';
 import type {WorkspaceSvg} from './workspace_svg.js';
 import * as Xml from './xml.js';
 
-export type FieldValidator<T = unknown> = (value: T) => void;
+export type FieldValidator<T = any> = (value?: T) => T|null|undefined;
 
 /**
  * Abstract class for an editable field.
  *
  * @alias Blockly.Field
  */
-export abstract class Field<T = unknown> implements IASTNodeLocationSvg,
-                                                    IASTNodeLocationWithBlock,
-                                                    IKeyboardAccessible,
-                                                    IRegistrable {
+export abstract class Field<T = any> implements IASTNodeLocationSvg,
+                                                IASTNodeLocationWithBlock,
+                                                IKeyboardAccessible,
+                                                IRegistrable {
   /**
    * To overwrite the default value which is set in **Field**, directly update
    * the prototype.