ymin() returns unexpected error for a waveform family

Hi Everyone,

I can not get the minimum value of a waveform family. I would expect that ymin returns a waveform where the Y values are minimum y values of the waveforms in the waveform family and the X values are the leaf values of the family. ymin returns with the error below for a family. I am able to plot the waveform family. ymin() work well for a different waveform family created by IS() function from a parametric DC sweep.

expression evaluation failed: Expression evaluates to nil
expression evaluation failed: ymin(-IF("/VD/PLUS"))

I created a simple procedure to check whether something is wrong with processing the family with a function, and it worked well. See code below. I am clueless how to debug the issue and generate the waveform, what I wanted to see, and which would help me make better decisions in the circuit design.

procedure( example(x)
   x+1
)
example(-ID("/VD/PLUS"))

Clearly there is something I do not notice or do not understand about using the function. Could you tell me how to debug it further and help me understand what's going on? Why is one waveform family accepted, but the other not.

Regards,
Zoltan