IF function don't comparison in Asterisk dialplan 1.8.11.0

this is my code:

[increment]
exten => s,1,Verbose(2,Increment the counter variable)

; Set the initial value of the variable
same => n,Set(CounterVariable=1)
same => n,Verbose(2,Current value of CounterVariable is: ${CounterVariable})

same => n,Set(IncrementValue=0)
; Now we can increment the value of CounterVariable if IncrementValue returns 1

same => n,Set(CounterVariable=${IF($[${IncrementValue} = 1]?${INC(CounterVariable)}:${CounterVariable})})

same => n,Verbose(2,Our IncrementValue returned: ${IncrementValue})

same => n,Verbose(2, Our new value of CounterVariable is: ${CounterVariable})

same => n,Hangup()

NOW My Question: i think the IF function don't work properly and ${CounterVariable} in end of dialplan return 2 only.why?

Thank You.