[R] Urgent: conditional formula for nls

[Andrew Coop]

I have data vectors x and y both with 179 observations.  I'm trying to
fit a nonlinear model with five parameters using nls.  The formula is
only defined within a range of x-values, it should be zero otherwise,
thus my attempted use of ifelse:

> df<-data.frame(x,y)
> nlsfit<-nls(y~ifelse(x>m&x<m+abs(s),abs(a)*d*abs(s)*exp(-(1/2)*(g+d*log((x-m)/(-x+m+abs(s)))^2))/(sqrt(2*pi)*(x-m)*(-x+m+abs(s))),0),data=df,start=list(a=0.37,g=0,d=1,m=0.42,s=0.24))
Error in nlsModel(formula, mf, start, wts) :
  singular gradient matrix at initial parameter estimates
In addition: Warning messages:
1: In log((x - m)/(-x + m + abs(s))) : NaNs produced
2: In log((x - m)/(-x + m + abs(s))) : NaNs produced
3: In log((x - m)/(-x + m + abs(s))) : NaNs produced
4: In log((x - m)/(-x + m + abs(s))) : NaNs produced
5: In log((x - m)/(-x + m + abs(s))) : NaNs produced
6: In log((x - m)/(-x + m + abs(s))) : NaNs produced
7: In log((x - m)/(-x + m + abs(s))) : NaNs produced

What am I doing wrong?

Thanks,
Andy