PandasNotImplementedError: The method `pd.Series.__iter__()` is not implemented. If you want to collect your data as an NumPy array

Question

I try to create a new column in Koalas dataframe df. The dataframe has 2 columns: col1 and col2. I need to create a new column newcol as a median of col1 and col2 values.

import numpy as np
import databricks.koalas as ks

# df is Koalas dataframe
df = df.assign(newcol=lambda x: np.median(x.col1, x.col2).astype(float))

But I get the following error:

PandasNotImplementedError: The method pd.Series.__iter__() is not implemented. If you want to collect your data as an NumPy array, use 'to_numpy()' instead.

Also I tried:

df.newcol = df.apply(lambda x: np.median(x.col1, x.col2), axis=1)

But it didn't work.