diff --git a/src/ar/cbu.spec.ts b/src/ar/cbu.spec.ts
index 37458070..1e94f09f 100644
--- a/src/ar/cbu.spec.ts
+++ b/src/ar/cbu.spec.ts
@@ -25,4 +25,25 @@ describe('ar/cbu', () => {
 
     expect(result.error).toBeInstanceOf(InvalidChecksum);
   });
+
+  // Regression: a check digit of 0 must be accepted. `10 - weightedSum(...)` yields
+  // 10 (not 0) when the weighted sum is 0, so without the `% 10` wrap these valid
+  // numbers were wrongly rejected as InvalidChecksum.
+  it('validate:0720429088000002339140', () => {
+    const result = validate('0720429088000002339140');
+
+    expect(result.isValid && result.compact).toEqual('0720429088000002339140');
+  });
+
+  it('validate:0000000000000000000000', () => {
+    const result = validate('0000000000000000000000');
+
+    expect(result.isValid && result.compact).toEqual('0000000000000000000000');
+  });
+
+  it('validate:0720429088000002339141', () => {
+    const result = validate('0720429088000002339141');
+
+    expect(result.error).toBeInstanceOf(InvalidChecksum);
+  });
 });
diff --git a/src/ar/cbu.ts b/src/ar/cbu.ts
index 6e3a7c22..6deb225d 100644
--- a/src/ar/cbu.ts
+++ b/src/ar/cbu.ts
@@ -63,20 +63,22 @@ const impl: Validator = {
     const [front, c1, back, c2] = strings.splitAt(value, 7, 8, 21);
 
     const s1 = String(
-      10 -
+      (10 -
         weightedSum(front, {
           reverse: true,
           weights: [3, 1, 7, 9, 3, 1, 7],
           modulus: 10,
-        }),
+        })) %
+        10,
     );
     const s2 = String(
-      10 -
+      (10 -
         weightedSum(back, {
           reverse: true,
           weights: [3, 1, 7, 9, 3, 1, 7, 9, 3, 1, 7, 9, 3, 1],
           modulus: 10,
-        }),
+        })) %
+        10,
     );
 
     if (s1 !== c1 || s2 !== c2) {