Move column to the right if it starts with a letter

[mfernandes]

I want to move all columns to the right if my row starts with a letter, not a number.
Dataframe that I have:

var1  var2  var3 ...
9       3       3
4       4       3
ABC   5

Dataframe that I want:

var1  var2  var3 ...
9       3       3
4       4       3
NaN   ABC   5

My code so far:

import pandas as pd
df=pd.read_csv("input.csv", sep=",")

mylist=['A', 'B', 'C', 'D'] # the alphabet list
# identify rows that start with a letter
m = df['NIF (EO)'].str.startswith(mylist)
# define columns to correct 
cols = df.columns[1:]
df.loc[m, cols] = df.loc[m, cols].shift(axis=1)

However, in the last line of the code I am obtaining the error:
"None of [Float64Index([nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,\n ...\n nan, nan, nan, nan, nan, nan, nan, nan, nan, nan],\n dtype='float64', length=200362)] are in the [index]"
What am I doing wrong?