diff --git a/contracts/src/helpers/PBHExternalNullifier.sol b/contracts/src/helpers/PBHExternalNullifier.sol
index f39b27c4..5a1d942e 100644
--- a/contracts/src/helpers/PBHExternalNullifier.sol
+++ b/contracts/src/helpers/PBHExternalNullifier.sol
@@ -15,7 +15,6 @@ import "@BokkyPooBahsDateTimeLibrary/BokkyPooBahsDateTimeLibrary.sol";
 ///      - Bits 0-7: Version
 
 //TODO: move this to a lib dir
-// TODO:
 library PBHExternalNullifier {
     /// @notice Thrown when the provided external nullifier doesn't
     /// contain the correct leading zeros
diff --git a/contracts/test/PBHExternalNullifier.t.sol b/contracts/test/PBHExternalNullifier.t.sol
index 995fe90e..b257df80 100644
--- a/contracts/test/PBHExternalNullifier.t.sol
+++ b/contracts/test/PBHExternalNullifier.t.sol
@@ -43,8 +43,11 @@ contract PBHExternalNullifierTest is Test {
         PBHExternalNullifier.verify(encoded, maxPbh);
     }
 
-    // TODO:
-    function testFuzz_verify_RevertIf_InvalidNullifierLeadingZeros() public {}
+    function testFuzz_verify_RevertIf_InvalidNullifierLeadingZeros(uint256 encoded) public {
+        vm.assume(encoded > type(uint40).max);
+        vm.expectRevert(PBHExternalNullifier.InvalidExternalNullifierLeadingZeros.selector);
+        PBHExternalNullifier.verify(encoded, 30);
+    }
 
     function testFuzz_verify_RevertIf_InvalidExternalNullifierVersion(uint8 pbhVersion) public {
         vm.assume(pbhVersion != PBHExternalNullifier.V1);