[R] query about the nlm function

[xuan ...]

Hi, I am a beginner to the R program and am currently having some problems 
with the nlm function. I have already checked that my function is a positive 
quadratic and since it has only one turning point (i plotted the graph to 
check) there should be no problems in terms of finding the minimum point. 
However, I keep getting error messages saying :
Error in nlm(poi.loglike, p = 0.3) : invalid function value in 'nlm' 
optimizer

Why does this error message appear and what are the usual causes of this 
error?
My primary aim is to calculate the negative poisson log likelihood function 
and by using the nlm function, to find the  maximum likelihood estimate.

Thanks to all who take their time to read and hopefully enlighten me with 
regards to this simple question.