Execute multiple requests in transaction error in crm 2015

i developed an application using the CRM 2015 SDK(v7.1.2). i want to send 2 requests for crm by ExecuteTransactionRequest, but i got an error:

The formatter threw an exception while trying to deserialize the message: There was an error while trying to deserialize parameterschemas.microsoft.com/.../Services:request. The
 InnerException message was 'Error in line 1 position 451. Element'schemas.microsoft.com/.../Services:request'
contains data from a type that maps to the name'schemas.microsoft.com/.../Contracts:ExecuteTransactionRequest'.
 The deserializer has no knowledge of any type that maps to this name.
Consider changing the implementation of the ResolveName method on your
DataContractResolver to return a non-null value for name'ExecuteTransactionRequest' and namespace'schemas.microsoft.com/.../Contracts'.'

i used this link: https://msdn.microsoft.com/en-us/library/mt634414.aspx

please help me!!!