diff --git a/src/format.spec.ts b/src/format.spec.ts
index a613e76..e64c0a9 100644
--- a/src/format.spec.ts
+++ b/src/format.spec.ts
@@ -5,10 +5,7 @@ describe('format(credentials)', function () {
   describe('arguments', function () {
     describe('credentials', function () {
       it('should be required', function () {
-        assert.throws(
-          () => (format as any)(),
-          /argument credentials is required/,
-        );
+        assert.throws(() => (format as any)(), /Expected an object/);
       });
 
       it('should accept credentials', function () {
@@ -17,65 +14,56 @@ describe('format(credentials)', function () {
       });
 
       it('should reject null', function () {
-        assert.throws(
-          format.bind(null, null as any),
-          /argument credentials is required/,
-        );
+        assert.throws(format.bind(null, null as any), /Expected an object/);
       });
 
       it('should reject a number', function () {
-        assert.throws(
-          format.bind(null, 42 as any),
-          /argument credentials is required/,
-        );
+        assert.throws(format.bind(null, 42 as any), /Expected an object/);
       });
 
       it('should reject a string', function () {
-        assert.throws(
-          format.bind(null, '' as any),
-          /argument credentials is required/,
-        );
+        assert.throws(format.bind(null, '' as any), /Expected an object/);
       });
 
       it('should reject an object without name', function () {
         assert.throws(
           format.bind(null, { pass: 'bar' } as any),
-          /argument credentials is required to have name and pass properties/,
+          /Object must have string properties "name" and "pass"/,
         );
       });
 
       it('should reject an object without pass', function () {
         assert.throws(
           format.bind(null, { name: 'foo' } as any),
-          /argument credentials is required to have name and pass properties/,
+          /Object must have string properties "name" and "pass"/,
         );
       });
 
       it('should reject an object with non-string name', function () {
         assert.throws(
           format.bind(null, { name: 42, pass: 'bar' } as any),
-          /argument credentials is required to have name and pass properties/,
+          /Object must have string properties "name" and "pass"/,
         );
       });
 
       it('should reject an object with non-string pass', function () {
         assert.throws(
           format.bind(null, { name: 'foo', pass: 42 } as any),
-          /argument credentials is required to have name and pass properties/,
+          /Object must have string properties "name" and "pass"/,
         );
       });
 
       it('should reject userid containing colon', function () {
         assert.throws(
           format.bind(null, { name: 'foo:bar', pass: 'baz' }),
-          /must not contain a colon or control characters/,
+          /must not contain a colon/,
         );
       });
 
       it('should reject control chars in userid', function () {
         assert.throws(
           format.bind(null, { name: 'foo\u0000bar', pass: 'baz' }),
-          /must not contain a colon or control characters/,
+          /must not contain control characters/,
         );
       });
 
@@ -96,21 +84,21 @@ describe('format(credentials)', function () {
   });
 
   describe('with empty password', function () {
-    it('should throw', function () {
+    it('should return header', function () {
       const header = format({ name: 'foo', pass: '' });
       assert.strictEqual(header, 'Basic Zm9vOg==');
     });
   });
 
   describe('with empty userid', function () {
-    it('should throw', function () {
+    it('should return header', function () {
       const header = format({ name: '', pass: 'pass' });
       assert.strictEqual(header, 'Basic OnBhc3M=');
     });
   });
 
   describe('with empty userid and pass', function () {
-    it('should throw', function () {
+    it('should return header', function () {
       const header = format({ name: '', pass: '' });
       assert.strictEqual(header, 'Basic Og==');
     });
diff --git a/src/index.ts b/src/index.ts
index d4d4b1b..959b832 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -26,7 +26,7 @@ export interface Credentials {
 
 export function parse(string: string): Credentials | undefined {
   if (typeof string !== 'string') {
-    return undefined;
+    throw new TypeError('Expected a string');
   }
 
   // parse header
@@ -52,39 +52,31 @@ export function parse(string: string): Credentials | undefined {
  * @public
  */
 export function format(credentials: Credentials): string {
-  if (!credentials) {
-    throw new TypeError('argument credentials is required');
-  }
-
-  if (typeof credentials !== 'object') {
-    throw new TypeError('argument credentials is required to be an object');
+  if (typeof credentials !== 'object' || credentials === null) {
+    throw new TypeError('Expected an object');
   }
 
   if (
     typeof credentials.name !== 'string' ||
     typeof credentials.pass !== 'string'
   ) {
-    throw new TypeError(
-      'argument credentials is required to have name and pass properties',
-    );
+    throw new TypeError('Object must have string properties "name" and "pass"');
   }
 
-  if (
-    credentials.name.includes(':') || // RFC 7617 disallows colon in username
-    CONTROL_CHARS_REGEXP.test(credentials.name)
-  ) {
-    throw new TypeError(
-      'argument credentials.name must not contain a colon or control characters',
-    );
+  // RFC 7617 disallows colon in username
+  if (credentials.name.includes(':')) {
+    throw new TypeError('Object "name" must not contain a colon');
   }
 
-  if (CONTROL_CHARS_REGEXP.test(credentials.pass)) {
+  const str = credentials.name + ':' + credentials.pass;
+
+  if (CONTROL_CHARS_REGEXP.test(str)) {
     throw new TypeError(
-      'argument credentials.pass must not contain control characters',
+      'Object "name" and "pass" must not contain control characters',
     );
   }
 
-  return 'Basic ' + base64.encode(credentials.name + ':' + credentials.pass);
+  return 'Basic ' + base64.encode(str);
 }
 
 /**
diff --git a/src/parse.spec.ts b/src/parse.spec.ts
index 6347ea1..5bd5b79 100644
--- a/src/parse.spec.ts
+++ b/src/parse.spec.ts
@@ -2,9 +2,9 @@ import { describe, it, assert } from 'vitest';
 import { parse } from './index.js';
 
 describe('parse(string)', function () {
-  describe('with undefined string', function () {
-    it('should return undefined', function () {
-      assert.strictEqual((parse as any)(), undefined);
+  describe('with non string', function () {
+    it('should throw', function () {
+      assert.throws(() => parse(undefined as any), /Expected a string/);
     });
   });