diff --git a/types/yup/index.d.ts b/types/yup/index.d.ts
index f29fe60d23..debbec3ada 100644
--- a/types/yup/index.d.ts
+++ b/types/yup/index.d.ts
@@ -369,8 +369,19 @@ export class Ref {
 // tslint:disable-next-line:no-empty-interface
 export interface Lazy extends Schema<any> {}
 
+export interface FormatErrorParams {
+  path: string;
+  type: string;
+  value?: any;
+  originalValue?: any;
+}
+
+export type LocaleValue =
+    | string
+    | ((params: FormatErrorParams) => string);
+
 export interface LocaleObject {
-    mixed?: { [key in keyof MixedSchema]?: string };
+    mixed?: { [key in keyof MixedSchema]?: string; } & { notType?: LocaleValue };
     string?: { [key in keyof StringSchema]?: string };
     number?: { [key in keyof NumberSchema]?: string };
     boolean?: { [key in keyof BooleanSchema]?: string };
diff --git a/types/yup/yup-tests.ts b/types/yup/yup-tests.ts
index ff45c00a92..2e3d854ec0 100644
--- a/types/yup/yup-tests.ts
+++ b/types/yup/yup-tests.ts
@@ -13,7 +13,8 @@ import {
     TestOptions,
     ValidateOptions,
     NumberSchema,
-    TestContext
+    TestContext,
+    LocaleObject
 } from "yup";
 
 // reach function
@@ -452,6 +453,33 @@ const validateOptions: ValidateOptions = {
     }
 };
 
+const localeNotType1: LocaleObject = {
+    mixed: {
+        required: "message",
+        notType: "message"
+    }
+};
+const localeNotType2: LocaleObject = {
+    mixed: {
+        required: "message",
+        notType: () => "message"
+    }
+};
+const localeNotType3: LocaleObject = {
+    mixed: {
+        required: "message",
+        notType: (_ref) => {
+            const isCast: boolean = _ref.originalValue != null && _ref.originalValue !== _ref.value;
+            const finalPartOfTheMessage = isCast
+                ? ` (cast from the value '${_ref.originalValue}').`
+                : '.';
+
+            return `${_ref.path} must be a '${_ref.type}'` +
+                ` but the final value was: '${_ref.value}'${finalPartOfTheMessage}`;
+        }
+    }
+};
+
 yup.setLocale({
     number: { max: "Max message", min: "Min message" },
     string: { email: "String message" }