integrate the PDF of normal distribution

Hi group,
What is the prpper way to integrate the PDF of normal distribution?
I tried:
(4) -> integrate(x/(sigma*sqrt(2*%pi))*exp(-(x-mu)**2/(2*sigma**2)),x=%minusInfinity..%plusInfinity)
(4) ->
   (4)  "failed"
                                                Type: Union(fail: failed,...)
But if I give a value to sigma, then it works:

(9) -> sigma:=10
(9) ->
   (9)  10
(11) -> integrate(x/(sigma*sqrt(2*%pi))*exp(-(x-mu)**2/(2*sigma**2)),x=%minusInfinity..%plusInfinity)
(11) ->
              +---+
          2mu\|%pi
   (11)  -----------
          +-+ +----+
         \|2 \|2%pi
                    Type: Union(f1: OrderedCompletion Expression Integer,...)

Is it possible for axiom to work without specify a value to sigma?
Thanks,
Ming