diff --git a/as/mod.ts b/as/mod.ts
index cc78ac7..6b946e8 100644
--- a/as/mod.ts
+++ b/as/mod.ts
@@ -32,7 +32,7 @@ export const as: {
    * });
    * const a: unknown = {};
    * if (isMyType(a)) {
-   *   const _: {foo?: string} = a;
+   *   const _: {foo?: string | undefined} = a;
    * }
    * ```
    */
diff --git a/as/optional.ts b/as/optional.ts
index 53ea462..937c828 100644
--- a/as/optional.ts
+++ b/as/optional.ts
@@ -28,7 +28,7 @@ import {
  * });
  * const a: unknown = {};
  * if (isMyType(a)) {
- *   const _: {foo?: string} = a;
+ *   const _: {foo?: string | undefined} = a;
  * }
  * ```
  */
diff --git a/as/optional_test.ts b/as/optional_test.ts
index 4fe66bf..8516acb 100644
--- a/as/optional_test.ts
+++ b/as/optional_test.ts
@@ -34,7 +34,12 @@ Deno.test("asOptional<T>", async (t) => {
     await t.step("predicated type is correct", () => {
       const v: unknown = undefined;
       if (pred(v)) {
-        assertType<Equal<typeof v, { a: number; b?: number; c?: number }>>(
+        assertType<
+          Equal<
+            typeof v,
+            { a: number; b?: number | undefined; c?: number | undefined }
+          >
+        >(
           true,
         );
       }
@@ -71,7 +76,10 @@ Deno.test("asOptional<T>", async (t) => {
       const v: unknown = undefined;
       if (pred(v)) {
         assertType<
-          Equal<typeof v, [number, number?, number?]>
+          Equal<
+            typeof v,
+            [number, (number | undefined)?, (number | undefined)?]
+          >
         >(
           true,
         );
diff --git a/is/mod.ts b/is/mod.ts
index 59c77da..0fd6be4 100644
--- a/is/mod.ts
+++ b/is/mod.ts
@@ -492,7 +492,7 @@ export const is: {
    * ] as const);
    * const a: unknown = [0, undefined, "a"];
    * if (isMyType(a)) {
-   *   const _: [number, string | undefined, boolean, number?, string?, boolean?] = a;
+   *   const _: [number, string | undefined, boolean, (number | undefined)?, (string | undefined)?, (boolean | undefined)?] = a;
    * }
    * ```
    *
@@ -511,7 +511,7 @@ export const is: {
    * );
    * const a: unknown = [0, "a", true, 0, 1, 2];
    * if (isMyType(a)) {
-   *   const _: [number, string?, boolean?, ...number[]] = a;
+   *   const _: [number, (string | undefined)?, (boolean | undefined)?, ...number[]] = a;
    * }
    * ```
    *
@@ -525,7 +525,7 @@ export const is: {
    * const isMyType = is.ParametersOf(predTup);
    * const a: unknown = [0, "a"];
    * if (isMyType(a)) {
-   *   const _: [number, string, boolean?] = a;
+   *   const _: [number, string, (boolean | undefined)?] = a;
    * }
    * ```
    */
@@ -766,7 +766,7 @@ export const is: {
    * }));
    * const a: unknown = { a: 0, b: "b", c: true, other: "other" };
    * if (isMyType(a)) {
-   *   const _: { a: number; b: string | undefined; c: boolean } = a;
+   *   const _: { a: number; b: string | undefined; c: boolean | undefined } = a;
    * }
    * ```
    */
diff --git a/is/object_of_test.ts b/is/object_of_test.ts
index 514ef4b..38eca30 100644
--- a/is/object_of_test.ts
+++ b/is/object_of_test.ts
@@ -89,10 +89,10 @@ Deno.test("isObjectOf<T>", async (t) => {
         Equal<
           typeof a,
           {
-            readonly a?: string;
-            readonly b?: string;
+            readonly a?: string | undefined;
+            readonly b?: string | undefined;
             readonly c: string;
-            d?: string;
+            d?: string | undefined;
             e: string;
             f: string;
           }
@@ -256,10 +256,10 @@ Deno.test("isObjectOf<T>", async (t) => {
           Equal<
             typeof x,
             {
-              readonly [a]?: string;
-              readonly [b]?: string;
+              readonly [a]?: string | undefined;
+              readonly [b]?: string | undefined;
               readonly [c]: string;
-              [d]?: string;
+              [d]?: string | undefined;
               [e]: string;
               [f]: string;
             }
diff --git a/is/parameters_of.ts b/is/parameters_of.ts
index 39134dd..d89951e 100644
--- a/is/parameters_of.ts
+++ b/is/parameters_of.ts
@@ -25,7 +25,7 @@ import { isArray } from "./array.ts";
  * ] as const);
  * const a: unknown = [0, undefined, "a"];
  * if (isMyType(a)) {
- *   const _: [number, string | undefined, boolean, number?, string?, boolean?] = a;
+ *   const _: [number, string | undefined, boolean, (number | undefined)?, (string | undefined)?, (boolean | undefined)?] = a;
  * }
  * ```
  *
@@ -44,7 +44,7 @@ import { isArray } from "./array.ts";
  * );
  * const a: unknown = [0, "a", true, 0, 1, 2];
  * if (isMyType(a)) {
- *   const _: [number, string?, boolean?, ...number[]] = a;
+ *   const _: [number, (string | undefined)?, (boolean | undefined)?, ...number[]] = a;
  * }
  * ```
  *
@@ -58,7 +58,7 @@ import { isArray } from "./array.ts";
  * const isMyType = is.ParametersOf(predTup);
  * const a: unknown = [0, "a"];
  * if (isMyType(a)) {
- *   const _: [number, string, boolean?] = a;
+ *   const _: [number, string, (boolean | undefined)?] = a;
  * }
  * ```
  */
diff --git a/is/parameters_of_test.ts b/is/parameters_of_test.ts
index 6bf5840..bbeea8d 100644
--- a/is/parameters_of_test.ts
+++ b/is/parameters_of_test.ts
@@ -55,7 +55,15 @@ Deno.test("isParametersOf<T>", async (t) => {
     const a: unknown = [0, "a"];
     if (isParametersOf(predTup)(a)) {
       assertType<
-        Equal<typeof a, [number | undefined, string, string?, boolean?]>
+        Equal<
+          typeof a,
+          [
+            number | undefined,
+            string,
+            (string | undefined)?,
+            (boolean | undefined)?,
+          ]
+        >
       >(true);
     }
   });
@@ -160,7 +168,13 @@ Deno.test("isParametersOf<T, R>", async (t) => {
       assertType<
         Equal<
           typeof a,
-          [number | undefined, string, string?, boolean?, ...number[]]
+          [
+            number | undefined,
+            string,
+            (string | undefined)?,
+            (boolean | undefined)?,
+            ...number[],
+          ]
         >
       >(
         true,
diff --git a/is/partial_of_test.ts b/is/partial_of_test.ts
index e0433a9..400d624 100644
--- a/is/partial_of_test.ts
+++ b/is/partial_of_test.ts
@@ -42,7 +42,14 @@ Deno.test("isPartialOf<T>", async (t) => {
       assertType<
         Equal<
           typeof a,
-          Partial<{ a: number; b: string; c: boolean; readonly d: string }>
+          Partial<
+            {
+              a: number;
+              b: string | undefined;
+              c: boolean | undefined;
+              readonly d: string;
+            }
+          >
         >
       >(true);
     }
@@ -89,8 +96,8 @@ Deno.test("isPartialOf<T>", async (t) => {
             typeof a,
             Partial<{
               a: number;
-              [b]: string;
-              [c]: boolean;
+              [b]: string | undefined;
+              [c]: boolean | undefined;
               readonly [d]: string;
             }>
           >
diff --git a/is/required_of.ts b/is/required_of.ts
index 77fc75f..e602091 100644
--- a/is/required_of.ts
+++ b/is/required_of.ts
@@ -30,7 +30,7 @@ import { isObjectOf } from "./object_of.ts";
  * }));
  * const a: unknown = { a: 0, b: "b", c: true, other: "other" };
  * if (isMyType(a)) {
- *   const _: { a: number; b: string | undefined; c: boolean } = a;
+ *   const _: { a: number; b: string | undefined; c: boolean | undefined } = a;
  * }
  * ```
  */
diff --git a/is/required_of_test.ts b/is/required_of_test.ts
index 9cb4e40..a460130 100644
--- a/is/required_of_test.ts
+++ b/is/required_of_test.ts
@@ -52,7 +52,12 @@ Deno.test("isRequiredOf<T>", async (t) => {
       assertType<
         Equal<
           typeof a,
-          { a: number; b: string | undefined; c: boolean; readonly d: string }
+          {
+            a: number;
+            b: string | undefined;
+            c: boolean | undefined;
+            readonly d: string;
+          }
         >
       >(true);
     }
@@ -113,7 +118,7 @@ Deno.test("isRequiredOf<T>", async (t) => {
             {
               a: number;
               [b]: string | undefined;
-              [c]: boolean;
+              [c]: boolean | undefined;
               readonly [d]: string;
             }
           >
diff --git a/is/strict_of_test.ts b/is/strict_of_test.ts
index 97cbeca..d8f5b11 100644
--- a/is/strict_of_test.ts
+++ b/is/strict_of_test.ts
@@ -150,7 +150,10 @@ Deno.test("isStrictOf<T>", async (t) => {
       const a: unknown = { a: 0, b: "a" };
       if (isStrictOf(is.ObjectOf(predObj))(a)) {
         assertType<
-          Equal<typeof a, { a: number; b: string | undefined; c?: boolean }>
+          Equal<
+            typeof a,
+            { a: number; b: string | undefined; c?: boolean | undefined }
+          >
         >(true);
       }
     });
@@ -258,7 +261,10 @@ Deno.test("isStrictOf<T>", async (t) => {
       const a: unknown = { a: 0, [b]: "a" };
       if (isStrictOf(is.ObjectOf(predObj))(a)) {
         assertType<
-          Equal<typeof a, { a: number; [b]: string | undefined; [c]?: boolean }>
+          Equal<
+            typeof a,
+            { a: number; [b]: string | undefined; [c]?: boolean | undefined }
+          >
         >(true);
       }
     });
diff --git a/type.ts b/type.ts
index e53d27e..58bd4b1 100644
--- a/type.ts
+++ b/type.ts
@@ -7,7 +7,7 @@
  * type Person = {
  *   name: string;
  *   age: number;
- *   address?: string;
+ *   address?: string | undefined;
  * };
  * const isPerson = is.ObjectOf({
  *   name: is.String,
@@ -34,7 +34,7 @@ export type Predicate<T> = (x: unknown) => x is T;
  * // type Person = {
  * //   name: string;
  * //   age: number;
- * //   address?: string;
+ * //   address?: string | undefined;
  * // };
  * ```
  */