Find the order position of a particular value in the pandas dataframe?

import pandas as pd


a = pd.DataFrame({'value': [0, 1, float('nan'), 3, 4, float('nan')]

I would like to find a location with the nan value in a above. indexWithout using the value

First of all, a['value'].Find the nan value with isnull() I'm trying to find the location (order) where the nan value is located, but I don't know how to find the last location. Of course, I can make the code complicated, but it's getting too long, so I'm asking if there's a simple way.