[R] newbie question: fitting power law

[Thomas Hoffmann]

Dear R-Helpers,

I try  to fit my x and y vector-data with a power law using a the 
following command:

 test <- nls(y ~ A*x^B, xy, start=list(A=0.5,B=0.8))

and I get the error message:

Error in numericDeriv(form[[3]], names(ind), env) :
        Missing value or an Infinity produced when evaluating the model

Does anybody know whats wrong? (it´s probably a simple newbie-error)

Thanks in advance

Thomas H.