diff --git a/src/pl/nip.spec.ts b/src/pl/nip.spec.ts
index 7003e40..11ff364 100644
--- a/src/pl/nip.spec.ts
+++ b/src/pl/nip.spec.ts
@@ -14,6 +14,12 @@ describe('pl/nip', () => {
     expect(result.isValid && result.compact).toEqual('8567346215');
   });
 
+  it('validate:0000002000', () => {
+    const result = validate('0000002000');
+
+    expect(result.error).toBeInstanceOf(InvalidChecksum);
+  });
+
   it('validate:12345678', () => {
     const result = validate('12345678');
 
diff --git a/src/pl/nip.ts b/src/pl/nip.ts
index 66093a4..fd5a34d 100644
--- a/src/pl/nip.ts
+++ b/src/pl/nip.ts
@@ -5,6 +5,7 @@
  * a straightforward weighted checksum.
  *
  * Source
+ *   https://www.oecd.org/tax/automatic-exchange/crs-implementation-and-assistance/tax-identification-numbers/Poland-TIN.pdf
  *
  * ENTITY
  */