diff --git a/library/CHANGELOG.md b/library/CHANGELOG.md
index c7f435d64..e67929991 100644
--- a/library/CHANGELOG.md
+++ b/library/CHANGELOG.md
@@ -5,6 +5,7 @@ All notable changes to the library will be documented in this file.
 ## vX.X.X (Month DD, YYYY)
 
 - Add `cuid2` validation (pull request #130)
+- Fix bug in `strict` and `strictAsync` method for optional keys (issue #131)
 
 ## v0.13.1 (August 23, 2023)
 
diff --git a/library/src/methods/strict/strict.test.ts b/library/src/methods/strict/strict.test.ts
index 6456c7e62..5c4f84367 100644
--- a/library/src/methods/strict/strict.test.ts
+++ b/library/src/methods/strict/strict.test.ts
@@ -1,20 +1,37 @@
 import { describe, expect, test } from 'vitest';
-import { number, object, string } from '../../schemas/index.ts';
-import { parse } from '../parse/index.ts';
+import { number, object, optional, string } from '../../schemas/index.ts';
 import { strict } from './strict.ts';
 
 describe('strict', () => {
   test('should detect unknown keys', () => {
-    const schema = strict(object({ key1: string(), key2: number() }));
+    const schema = strict(object({ key1: string(), key2: optional(number()) }));
 
     const input1 = { key1: 'test', key2: 123 };
-    const output1 = parse(schema, input1);
-    expect(output1).toEqual(input1);
-
-    const keysError = 'Invalid keys';
-    const input2 = { key1: 'test', key2: 123, key3: 'unknown' };
-    expect(() => parse(schema, input2)).toThrowError(keysError);
-    const input3 = { key1: 'test', key2: 123, key3: 'unknown', key4: 123 };
-    expect(() => parse(schema, input3)).toThrowError(keysError);
+    const result1 = schema._parse(input1);
+    expect(result1.output).toEqual(input1);
+
+    const input2 = { key1: 'test', key2: undefined };
+    const result2 = schema._parse(input2);
+    expect(result2.output).toEqual(input2);
+
+    const input3 = { key1: 'test' };
+    const result3 = schema._parse(input3);
+    expect(result3.output).toEqual(input3);
+
+    const input4 = { key1: 'test', key2: 123, key3: '' };
+    const result4 = schema._parse(input4);
+    expect(result4.issues?.length).toBe(1);
+
+    const input5 = { key1: 'test', key2: undefined, key3: '' };
+    const result5 = schema._parse(input5);
+    expect(result5.issues?.length).toBe(1);
+
+    const input6 = { key1: 'test', key3: '' };
+    const result6 = schema._parse(input6);
+    expect(result6.issues?.length).toBe(1);
+
+    const input7 = { key1: 'test', key2: 123, key3: '', key4: '' };
+    const result7 = schema._parse(input7);
+    expect(result7.issues?.length).toBe(1);
   });
 });
diff --git a/library/src/methods/strict/strict.ts b/library/src/methods/strict/strict.ts
index 8e244e354..da444e152 100644
--- a/library/src/methods/strict/strict.ts
+++ b/library/src/methods/strict/strict.ts
@@ -28,9 +28,7 @@ export function strict<TSchema extends ObjectSchema<any>>(
     _parse(input, info) {
       const result = schema._parse(input, info);
       return !result.issues &&
-        // Check length of input and output keys
-        Object.keys(input as object).length !==
-          Object.keys(result.output).length
+        Object.keys(input as object).some((key) => !(key in result.output))
         ? getIssues(info, 'object', 'strict', error || 'Invalid keys', input)
         : result;
     },
diff --git a/library/src/methods/strict/strictAsync.test.ts b/library/src/methods/strict/strictAsync.test.ts
index 149872020..1638c2ef5 100644
--- a/library/src/methods/strict/strictAsync.test.ts
+++ b/library/src/methods/strict/strictAsync.test.ts
@@ -1,20 +1,39 @@
 import { describe, expect, test } from 'vitest';
-import { number, objectAsync, string } from '../../schemas/index.ts';
-import { parseAsync } from '../parse/index.ts';
+import { number, objectAsync, optional, string } from '../../schemas/index.ts';
 import { strictAsync } from './strictAsync.ts';
 
 describe('strict', () => {
   test('should detect unknown keys', async () => {
-    const schema = strictAsync(objectAsync({ key1: string(), key2: number() }));
+    const schema = strictAsync(
+      objectAsync({ key1: string(), key2: optional(number()) })
+    );
 
     const input1 = { key1: 'test', key2: 123 };
-    const output1 = await parseAsync(schema, input1);
-    expect(output1).toEqual(input1);
-
-    const keysError = 'Invalid keys';
-    const input2 = { key1: 'test', key2: 123, key3: 'unknown' };
-    await expect(parseAsync(schema, input2)).rejects.toThrowError(keysError);
-    const input3 = { key1: 'test', key2: 123, key3: 'unknown', key4: 123 };
-    await expect(parseAsync(schema, input3)).rejects.toThrowError(keysError);
+    const result1 = await schema._parse(input1);
+    expect(result1.output).toEqual(input1);
+
+    const input2 = { key1: 'test', key2: undefined };
+    const result2 = await schema._parse(input2);
+    expect(result2.output).toEqual(input2);
+
+    const input3 = { key1: 'test' };
+    const result3 = await schema._parse(input3);
+    expect(result3.output).toEqual(input3);
+
+    const input4 = { key1: 'test', key2: 123, key3: '' };
+    const result4 = await schema._parse(input4);
+    expect(result4.issues?.length).toBe(1);
+
+    const input5 = { key1: 'test', key2: undefined, key3: '' };
+    const result5 = await schema._parse(input5);
+    expect(result5.issues?.length).toBe(1);
+
+    const input6 = { key1: 'test', key3: '' };
+    const result6 = await schema._parse(input6);
+    expect(result6.issues?.length).toBe(1);
+
+    const input7 = { key1: 'test', key2: 123, key3: '', key4: '' };
+    const result7 = await schema._parse(input7);
+    expect(result7.issues?.length).toBe(1);
   });
 });
diff --git a/library/src/methods/strict/strictAsync.ts b/library/src/methods/strict/strictAsync.ts
index d10e38b36..17eb24b68 100644
--- a/library/src/methods/strict/strictAsync.ts
+++ b/library/src/methods/strict/strictAsync.ts
@@ -28,9 +28,7 @@ export function strictAsync<TSchema extends ObjectSchemaAsync<any>>(
     async _parse(input, info) {
       const result = await schema._parse(input, info);
       return !result.issues &&
-        // Check length of input and output keys
-        Object.keys(input as object).length !==
-          Object.keys(result.output).length
+        Object.keys(input as object).some((key) => !(key in result.output))
         ? getIssues(info, 'object', 'strict', error || 'Invalid keys', input)
         : result;
     },
diff --git a/website/src/routes/guides/(main-concepts)/schemas/index.mdx b/website/src/routes/guides/(main-concepts)/schemas/index.mdx
index b8a3a2bb8..d51778439 100644
--- a/website/src/routes/guides/(main-concepts)/schemas/index.mdx
+++ b/website/src/routes/guides/(main-concepts)/schemas/index.mdx
@@ -57,7 +57,7 @@ const LoginSchema = object({
   ]),
   password: string('Your password must be a string.', [
     minLength(1, 'Please enter your password.'),
-    minLength(8, 'You password must have 8 characters or more.'),
+    minLength(8, 'Your password must have 8 characters or more.'),
   ]),
 });
 ```