diff --git a/src/lib/provable/string.ts b/src/lib/provable/string.ts
index 2a52e33cdf..1a4bd4d451 100644
--- a/src/lib/provable/string.ts
+++ b/src/lib/provable/string.ts
@@ -88,7 +88,7 @@ class CircuitString extends Struct(RawCircuitString) {
       let isNull = this.values[i].isNull();
       mask[i] = isNull.and(wasntNullAlready);
       wasntNullAlready = isNull.not().and(wasntNullAlready);
-      length.add(wasntNullAlready.toField());
+      length = length.add(wasntNullAlready.toField());
     }
     // mask has length n+1, the last element is true when `this` has no null char
     mask[n] = wasntNullAlready;
diff --git a/src/lib/provable/test/string.unit-test.ts b/src/lib/provable/test/string.unit-test.ts
index 8eccb4bee8..4360baddce 100644
--- a/src/lib/provable/test/string.unit-test.ts
+++ b/src/lib/provable/test/string.unit-test.ts
@@ -150,6 +150,18 @@ describe('Circuit String', () => {
     });
   });
 
+  describe('#length', () => {
+    test('length', async () => {
+      const str1 = CircuitString.fromString('abcd');
+      expect(str1.length()).toEqual(Field(4));
+
+      await Provable.runAndCheck(() => {
+        const str1 = CircuitString.fromString('abcd');
+        expect(str1.length()).toEqual(Field(4));
+      });
+    });
+  });
+
   /*   describe('CircuitString8', async () => {
     test('cannot create more than 8 chars', () => {
       expect(() => {