diff --git a/src/isValid.ts b/src/isValid.ts
index 67481d5..7516833 100644
--- a/src/isValid.ts
+++ b/src/isValid.ts
@@ -48,7 +48,8 @@ export function validatePreparseResult(pr: PreparseResult, options?: ParserOptio
   const year = toGregorianYear(pr.Y, options) ?? base.Y;
   const month = pr.M ?? base.M;
 
-  return pr._index > 0
+  return !pr._error
+    && pr._index > 0
     && pr._length > 0
     && pr._index === pr._length
     && pr._match > 0
diff --git a/src/preparse.ts b/src/preparse.ts
index db56443..3604f0a 100644
--- a/src/preparse.ts
+++ b/src/preparse.ts
@@ -21,6 +21,11 @@ export interface PreparseResult extends ParsedComponents {
    * Number of matched tokens
    */
   _match: number;
+
+  /**
+   * True when a required format token could not be matched.
+   */
+  _error?: true;
 }
 
 const wildcard = ' ';
@@ -70,6 +75,7 @@ export function preparse(dateString: string, arg: string | CompiledObject, optio
 
     if (result) {
       if (!result.length) {
+        pr._error = true;
         break;
       }
       if (result.token && validateToken(result.token)) {
@@ -77,7 +83,7 @@ export function preparse(dateString: string, arg: string | CompiledObject, optio
       }
       pr._index += result.length;
       pr._match++;
-    } else if (token === str[0] || token === wildcard) {
+    } else if (token === str[0] || token === wildcard && str.length > 0) {
       pr._index++;
     } else if (COMMENT.test(token) && !str.indexOf(token.replace(COMMENT, '$1'))) {
       pr._index += token.length - 2;
@@ -85,6 +91,7 @@ export function preparse(dateString: string, arg: string | CompiledObject, optio
       pr._index = dateString.length;
       break;
     } else {
+      pr._error = true;
       break;
     }
   }
diff --git a/tests/incomplete-format.spec.ts b/tests/incomplete-format.spec.ts
new file mode 100644
index 0000000..00ead39
--- /dev/null
+++ b/tests/incomplete-format.spec.ts
@@ -0,0 +1,49 @@
+import { describe, expect, test } from 'vitest';
+import { compile, isValid, parse, preparse } from '@/index.ts';
+import { parser } from '@/plugins/day-of-week.ts';
+
+describe('incomplete format matching', () => {
+  test.each([
+    ['2025', 'YYYY-MM-DD'],
+    ['2025-', 'YYYY-MM-DD'],
+    ['2025-01', 'YYYY-MM-DD'],
+    ['2025-01-', 'YYYY-MM-DD'],
+    ['2025-01-01', 'YYYY-MM-DD HH:mm'],
+    ['2025', 'YYYY[ required]'],
+    ['2025', 'YYYY '],
+    ['2025', 'YYYY ...'],
+    ['2025', 'YYYY  ...'],
+    ['2025 ', 'YYYY  ...']
+  ])('rejects %s when %s still requires input', (input, format) => {
+    for (const pattern of [format, compile(format)]) {
+      expect(isValid(input, pattern)).toBe(false);
+      expect(Number.isNaN(parse(input, pattern).getTime())).toBe(true);
+    }
+  });
+
+  test('rejects an absent plugin token', () => {
+    const options = { plugins: [parser] };
+    expect(isValid('2025-01-01 ', 'YYYY-MM-DD dddd', options)).toBe(false);
+    expect(Number.isNaN(parse('2025-01-01 ', 'YYYY-MM-DD dddd', options).getTime())).toBe(true);
+  });
+
+  test('preserves parsed components when matching fails', () => {
+    expect(preparse('2025-01', 'YYYY-MM-DD')).toMatchObject({
+      Y: 2025, M: 1, _error: true, _index: 7, _length: 7, _match: 2
+    });
+  });
+
+  test.each([
+    ['2025', 'YYYY'],
+    ['2025-01', 'YYYY-MM'],
+    ['2025-01-01', 'YYYY-MM-DD'],
+    ['2025', 'YYYY...'],
+    ['2025 additional text', 'YYYY...'],
+    ['2025 ', 'YYYY '],
+    ['2025 ', 'YYYY ...'],
+    ['2025  ', 'YYYY  ...']
+  ])('accepts %s when %s is fully matched', (input, pattern) => {
+    expect(isValid(input, pattern)).toBe(true);
+    expect(Number.isNaN(parse(input, pattern).getTime())).toBe(false);
+  });
+});
diff --git a/tests/preparse.spec.ts b/tests/preparse.spec.ts
index 7d9be04..c9813ac 100644
--- a/tests/preparse.spec.ts
+++ b/tests/preparse.spec.ts
@@ -130,7 +130,7 @@ describe('MMMM', () => {
   });
 
   test('Zero', () => {
-    const dt = { _index: 0, _length: 4, _match: 0 };
+    const dt = { _error: true, _index: 0, _length: 4, _match: 0 };
     expect(preparse('Zero', 'MMMM')).toEqual(dt);
   });
 });
@@ -147,7 +147,7 @@ describe('MMM', () => {
   });
 
   test('Zero', () => {
-    const dt = { _index: 0, _length: 4, _match: 0 };
+    const dt = { _error: true, _index: 0, _length: 4, _match: 0 };
     expect(preparse('Zero', 'MMM')).toEqual(dt);
   });
 });
@@ -300,7 +300,7 @@ describe('AA', () => {
   });
 
   test('M.M.', () => {
-    const dt = { _index: 0, _length: 4, _match: 0 };
+    const dt = { _error: true, _index: 0, _length: 4, _match: 0 };
     expect(preparse('M.M.', 'AA')).toEqual(dt);
   });
 });
@@ -317,7 +317,7 @@ describe('A', () => {
   });
 
   test('MM', () => {
-    const dt = { _index: 0, _length: 2, _match: 0 };
+    const dt = { _error: true, _index: 0, _length: 2, _match: 0 };
     expect(preparse('MM', 'A')).toEqual(dt);
   });
 });
@@ -334,7 +334,7 @@ describe('aa', () => {
   });
 
   test('m.m.', () => {
-    const dt = { _index: 0, _length: 4, _match: 0 };
+    const dt = { _error: true, _index: 0, _length: 4, _match: 0 };
     expect(preparse('m.m.', 'aa')).toEqual(dt);
   });
 });
@@ -351,7 +351,7 @@ describe('a', () => {
   });
 
   test('mm', () => {
-    const dt = { _index: 0, _length: 2, _match: 0 };
+    const dt = { _error: true, _index: 0, _length: 2, _match: 0 };
     expect(preparse('mm', 'a')).toEqual(dt);
   });
 });