NonParamDMLIV sometimes ZeroDivisionError

Seems that one can run into ZeroDivisionErrors when using NonParamDMLIV. Reproduction code below. Note, the error is not consistent. You may have to run it multiple times before getting the ZeroDivisionError. 

From briefly looking into it, it seems the T residuals are all 0's when the error occurs.

econml version 0.15.0b1. Haven't tried other versions.

Let me know if I'm missing something!

from econml.iv.dml import NonParamDMLIV
import numpy as np
from sklearn.linear_model import LinearRegression

n = 100
d_x = 3

Y = np.random.normal(size=(n,))
T = np.random.normal(size=(n,))
X = np.random.normal(size=(n, d_x))
Z = np.random.normal(size=(n,))

est = NonParamDMLIV(discrete_instrument=False, discrete_treatment=False, model_final=LinearRegression())

est.fit(Y, T, Z=Z, X=X)