diff --git a/docs/extending.md b/docs/extending.md
index 67e5cc432..0ab60bcd6 100644
--- a/docs/extending.md
+++ b/docs/extending.md
@@ -40,7 +40,7 @@ function parseDateFromFormats(formats, parseStrict) {
 yup.addMethod(yup.date, 'format', parseDateFromFormats);
 ```
 
-Note that `addMethod` isn't really magic, it mutates the prototype of the passed in schema.
+Note that `addMethod` isn't magic, it mutates the prototype of the passed in schema.
 
 > Note: if you are using TypeScript you also need to adjust the class or interface
 > see the [typescript](./typescript) docs for details.
@@ -49,7 +49,7 @@ Note that `addMethod` isn't really magic, it mutates the prototype of the passed
 
 If you're use case calls for creating an entirely new type. inheriting from
 and existing schema class may be best: Generally you should not inheriting from
-the abstract `Schema` unless you know what you are doing. The other types are fair game though.
+the abstract `BaseSchema` unless you know what you are doing. The other types are fair game though.
 
 You should keep in mind some basic guidelines when extending schemas:
 
diff --git a/docs/typescript.md b/docs/typescript.md
index b51754476..23bb104f5 100644
--- a/docs/typescript.md
+++ b/docs/typescript.md
@@ -118,3 +118,13 @@ declare module 'yup' {
 > Watch out!: If your method needs to adjust schema generics, you likely
 > need to also extend the Required*, and Defined* interfaces associated with
 > each basic type. Consult the core types for examples on how to do this
+
+Be careful of the yup type hierarchy as it's a bit tricky. All schema (including `mixed`)
+extend the abstract `BaseSchema` class.
+
+### Special note about MixedSchema and BaseSchema
+
+As far as typescript is concerned, `mixed` schema inherit from `BaseSchema` like other schema; all other schema do **not** extend `MixedSchema`. **In actuality** Mixed is an alias for BaseSchema, meaning `addMethod(mixed)` will add a new method to all schema.
+
+This means that type extensions to `mixed` should generally be put on `BaseSchema` if
+you want the method to be available to all sub classes.
diff --git a/src/mixed.ts b/src/mixed.ts
index 165d72130..1157f2c16 100644
--- a/src/mixed.ts
+++ b/src/mixed.ts
@@ -4,23 +4,11 @@ import { AnyObject, Maybe, Optionals } from './types';
 import type { Defined } from './util/types';
 import BaseSchema from './schema';
 
-export function create<TType = any>() {
-  return new MixedSchema<TType | undefined>();
-}
-
-export default class MixedSchema<
+declare class MixedSchema<
   TType = any,
   TContext = AnyObject,
   TOut = TType
-> extends BaseSchema<TType, TContext, TOut> {}
-
-create.prototype = MixedSchema.prototype;
-
-export default interface MixedSchema<
-  TType = any,
-  TContext = AnyObject,
-  TOut = TType
-> {
+> extends BaseSchema<TType, TContext, TOut> {
   default<TNextDefault extends Maybe<TType>>(
     def: TNextDefault | (() => TNextDefault),
   ): TNextDefault extends undefined
@@ -46,3 +34,13 @@ export default interface MixedSchema<
   nullable(isNullable?: true): MixedSchema<TType | null, TContext>;
   nullable(isNullable: false): MixedSchema<Exclude<TType, null>, TContext>;
 }
+
+const Mixed: typeof MixedSchema = BaseSchema as any;
+
+export default Mixed;
+
+export function create<TType = any>() {
+  return new Mixed<TType | undefined>();
+}
+// XXX: this is using the Base schema so that `addMethod(mixed)` works as a base class
+create.prototype = Mixed.prototype;
diff --git a/test/yup.js b/test/yup.js
index 05fdbb0f4..0b1621676 100644
--- a/test/yup.js
+++ b/test/yup.js
@@ -16,6 +16,8 @@ import {
   NumberSchema,
   BooleanSchema,
   DateSchema,
+  mixed,
+  MixedSchema,
 } from '../src';
 
 describe('Yup', function () {
@@ -191,6 +193,18 @@ describe('Yup', function () {
   });
 
   describe('addMethod', () => {
+    it('extending mixed should make method accessible everywhere', () => {
+      addMethod(mixed, 'foo', () => 'here');
+
+      expect(string().foo()).to.equal('here');
+    });
+
+    it('extending Mixed should make method accessible everywhere', () => {
+      addMethod(MixedSchema, 'foo', () => 'here');
+
+      expect(string().foo()).to.equal('here');
+    });
+
     test.each([
       ['object', object],
       ['array', array],