Implementing numpy.mean in Expression using Calculate Field Tool in ModelBuilder

I am using ArcGIS 10.4 ModelBuilder.
I developed a model that iterates through a bunch of feature datasets. I need to add a field to each one of these features and apply one formula on it. My formula consists on dividing one column by its mean.
I have implemented a formula on the "expression" part of the "Calculate Field" tool. As it is shown in the image:

The result of this is always 1. It seems that the mean is calculated per row and not for the whole column. So it is dividing the value by itself giving 1 as a result.

I have also tried to define my own mean, but so far with out success

def my_mean(sample):
 return sum(sample) / len(sample)

How should I proceed?