Scheduled Transactions Unparsable Formula

[子風]

GnuCash 2.6.12
Window 7 32bit

When I try to use Scheduled Transactions and make it more automatic.

So I modified the "fin.scm" to meet what I need.

But I can't do it. It always shows the error message "Couldn't parse 
redit-formula".

The credit-formula is "computeInterestIncrementFixed(10422:i)"

However, if I replaced "i" with "-i" like 
"computeInterestIncrementFixed(10422:-i)", it would be OK.

But It is not what I need.

There are source code as below. And I run it online working fine.

https://repl.it/CLmn/1

;; for fixed rate once in a year
(define ratePublic 0.0116)

(define (gnc:futureValueR a t)
     (if (<= t 0)
         a
         (round (* (+ 1 ratePublic) (gnc:futureValueR a (- t 1))))
      )
)

(define (gnc:computeInterestIncrementFixed a t)
   (let ((thisVal (gnc:futureValueR a t))
         (prevVal (gnc:futureValueR a (- t 1)))
        )
     (- thisVal prevVal)
   )
)