How to add an Exchange Rate UDF field in the voucher Level

I wanted to add an Exchange rate UDF field in the voucher Level. Once added I getting below error,

---------------------------
Error
---------------------------
Error in TDL.
'Field:BankExRateRupee'
Incorrect TYPE for Field!
Conflicts with STORAGE!
---------------------------
OK
---------------------------

Pls help me to resolve this issue



[#Part: VCH Narration] ;; Modify the Narration Part to add the details
Add : Option : Exchange VCH Narration : Not (@@IsDelNote or @@IsRcptNote or @@IsSalesOrder Or @@IsPurcOrder)
[!Part: Exchange VCH Narration ]

Add : Line : At Beginning : Exchange Rate Number

[Line: Exchange Rate Number]

Fields : Medium Prompt, BankExRateRupee
Local : Field : Medium Prompt : Info : $$LocaleString:"Rupee Exchange Rate:"
[Field:BankExRateRupee]

Use: Amount XchgRate Field
Storage: RsExchgeRate
Set as: $$GetLastUsedExchangeRate:"Rs"
Set Always: Yes
Format: "Decimal:4"
[System:UDF]

RsExchgeRate : Rate of Exchange: 1700