self-starting non linear model

Hi List!

I want to apply a selfstarting Michaelis-Menten-Model to my data: 
SSmicmen {stats}

I use thist function in the following way: 
model<-nls(distance~SSmicmen(watercontent, Vm, K))

When I want to compute it I get the error message:

                                                Fehler in lm.fit(x, y, 
offset = offset, singular.ok = singular.ok, ...) :
                                                NA/NaN/Inf in externem 
Funktionsaufruf (arg 4)

which would be in English: Error in lm.fit(x, y, offset = offset, 
singular.ok = singular.ok, ...) :
                                               NA/NaN/Inf in external 
function call(arg 4)

Does this mean the function just does not fit my data or is there 
anything I overlooked - any mistake I made defining the model???

Anna