    function transferVesting(address _grantor, address _beneficiary, uint256 _amount) external {
        require(
            msg.sender == tokenIssuer || msg.sender == manager || msg.sender == vestingDeployer,
            "SS_StepVesting: unauthorized"
        );
        require(_beneficiary != address(0), "SS_StepVesting: beneficiary is zero");
        require(_amount > 0, "SS_StepVesting: amount is zero");
        Vesting storage grantorVesting = _vestings[_grantor];
        require(
            grantorVesting.totalAmount - grantorVesting.amountClaimed >= _amount,
            "SS_StepVesting: insufficient balance"
        ); // 3.8. Claimed amount not checked in transferVesting function


        grantorVesting.totalAmount -= _amount;
        grantorVesting.releaseRate = grantorVesting.totalAmount / numOfSteps;


        _createVesting(_beneficiary, _amount, grantorVesting.stepsClaimed, true);


        emit VestingTransferred(_grantor, _beneficiary, _amount);
    }
    function transferVesting(address _grantor, address _beneficiary, uint256 _amount) external {
        require(
            msg.sender == tokenIssuer || msg.sender == manager || msg.sender == vestingDeployer,
            "SS_StepVesting: unauthorized"
        );
        require(_beneficiary != address(0), "SS_StepVesting: beneficiary is zero");
        require(_amount > 0, "SS_StepVesting: amount is zero");
        Vesting storage grantorVesting = _vestings[_grantor];
        require(
            grantorVesting.totalAmount - grantorVesting.amountClaimed >= _amount,
            "SS_StepVesting: insufficient balance"
        ); // 3.8. Claimed amount not checked in transferVesting function


        grantorVesting.totalAmount -= _amount;
        grantorVesting.releaseRate = grantorVesting.totalAmount / numOfSteps;
+       require(grantorVesting.releaseRate > 0, "SS_StepVesting: releaseRate is zero");

        _createVesting(_beneficiary, _amount, grantorVesting.stepsClaimed, true);


        emit VestingTransferred(_grantor, _beneficiary, _amount);
    }
