Error when Calculating Price Manullay

Hi

I have followed the article "Sample: Calculate Price plug-in" for quote entity, but I get following error when the code wants to update tax and total amount: 

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=7.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: Incorrect attribute value type System.Int32Detail:
<OrganizationServiceFault xmlns:i="www.w3.org/.../XMLSchema-instance" xmlns="schemas.microsoft.com/.../Contracts">
<ErrorCode>-2147220989</ErrorCode>
<ErrorDetails xmlns:d2p1="schemas.datacontract.org/.../System.Collections.Generic" />
<Message>Incorrect attribute value type System.Int32</Message>
<Timestamp>2017-01-16T19:51:59.932836Z</Timestamp>
<InnerFault i:nil="true" />
<TraceText>

[TaxCalculator: TaxCalculator.CalculatePricePlugin]
[4b8695e5-20dc-e611-93f6-000c29609d80: TaxCalculator.CalculatePricePlugin: CalculatePrice of quote]

[Microsoft.Crm.ObjectModel: Microsoft.Crm.ObjectModel.TargetAttributeValidationPlugin]
[fc743e7d-fbea-4695-bdb9-7d78334c8474: TargetAttributeValidationPlugin]

</TraceText>
</OrganizationServiceFault>