Error in Deploy Custom Service

i have created customService:

Service class has the method

[SysEntryPointAttribute(true),

AifCollectionTypeAttribute(’_payments’, Types::Class, Classstr(VendorPaymentContract)),

AifCollectionTypeAttribute(‘return’, Types::String)]

public str CreatePayments(List _payments)

{

// i have my code here

return “success or fail”;

}

My contract class is

[DataContractAttribute]

class VendorPaymentContract

{

AccountNum acc;

str JournalName;

List offsetDimension;

AmountCur amount;

str description ;

}

[DataMemberAttribute]

public List offsetDimension(List _offsetDimension = offsetDimension)

{

offsetDimension = _offsetDimension;

return offsetDimension;

}

When i am trying to deploy the servicegroup for the above service. i am getting error

" The service ‘VendorPaymentsListService’ could not be generated.\n Error: An attribute of type ‘AifCollectionTypeAttribute’ has not been defined. Method: ‘VendorPaymentContract.offsetDimension’, Parameter: ‘’."

i don’t know where/how to Specify AifCollectionTypeAttribute for offsetDimension.

Can anyone help me! Thanks