xmax and xmin are returning nil

Hi,

I am trying to run xmax function on a derived signal, and even though I can see a clear maximum in the plot the xmax(z 1) is returning nil.

The circuit I am using is a simple LC tank driven by a voltage source and I am measuring current and I am using the following outputs

i = ifreq('ac "/V4/PLUS")

z = 1/i

now whether i use xmax(z 1) or xmax(z 0), both return nil which is strange because a maximum does exist. similarly running xmin also returns nil.

My main goal is to find the resonance frequency of the LC tank and then use sensitivity analysis to see how much it is sensitive to individual components.

values of the design variables are

ls=400p, Q=200K, Q_cap=1K, var_cap=100f,w=2*3.14*30e9