"ylab" argument to plot.stepfun

Dear R Users,

Does anyone know how to specify Y-label in plot.stepfun()? I do:

library(stepfun)
x = ecdf(rnorm(30))
plot(x, ylab='CDF')

and I get:

Error in plot.stepfun(ylab = "Fn(x)", ..., verticals = verticals) :
  formal argument "ylab" matched by multiple actual arguments

Thanks,
Aleksey