A step too far in regression fit ?

[<pl...@pi...>]

Hi,

I have hit this in a couple of ways recently in cases where it seemed it 
should succeed.

The fit is basically making to a convergence , printing all the fitted 
values , stats , the lot but then failing. I am wondering if it is still 
necessary to it to do the invert RtR since it has done all I asked.



resultant parameter values

a0              = 384.634
b0              = 4850.85
c0              = 1e-30
pk0             = 9.10484

After 11 iterations the fit converged.
final sum of squares of residuals : 304.992
rel. change during last iteration : -1.04716e-06

degrees of freedom    (FIT_NDF)                        : 197
rms of residuals      (FIT_STDFIT) = sqrt(WSSR/ndf)    : 1.24426
variance of residuals (reduced chisquare) = WSSR/ndf   : 1.54818

          Singular matrix in Invert_RtR


In this case I had started with data where no extra offset was needed , 
ie c0=0 was correct before running fit.

I had a similar occurrence where another via param was at its final 
value and I got a singularity. If I deliberately initialised it to be a 
bit off, fit put it back where it was to start with without failing.

Is it correct for the method to succeed but fail :?

Thanks.