Error when trying to use seasonal_decompose from Statsmodels

Question

I have the following dataframe called 'data':

Month	Revenue Index
1920-01-01	1.72
1920-02-01	1.83
1920-03-01	1.94
...	...
2021-10-01	114.20
2021-11-01	115.94
2021-12-01	116.01

This is essentially a monthly revenue index on which I am trying to use seasonal_decompose with the following code:

result = seasonal_decompose(data['Revenue Index'], model='multiplicative')

But unfortunately I get the following error:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-39-08e3139bbf77> in <module>()
----> 1 result = seasonal_decompose(data['Consumptieprijsindex'], model='multiplicative')
      2 rcParams['figure.figsize'] = 12, 6
      3 plt.rc('lines', linewidth=1, color='r')
      4 
      5 fig = result.plot()

/usr/local/lib/python3.7/dist-packages/statsmodels/tsa/seasonal.py in seasonal_decompose(x, model, filt, freq, two_sided, extrapolate_trend)
    125             freq = pfreq
    126         else:
--> 127             raise ValueError("You must specify a freq or x must be a "
    128                              "pandas object with a timeseries index with "
    129                              "a freq not set to None")

ValueError: You must specify a freq or x must be a pandas object with a timeseries index with a freq not set to None

Does anyone know how to solve this issue? Thanks!

Answer 1

The following code in the comments answered my question:

result = seasonal_decompose(data['Revenue Index'], model='multiplicative', period=12)