• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of pandas (1.0.5)

series = pd.Series({'1':1,'2':None,'3':np.nan, '4':''})

will create a series which looks like:

1 1 2 None 3 NaN 4 dtype: object 

When using the fillna method on this series on index 3 it changes also the entry in index 2!

series.fillna({'3':0}) 

This will result in a output as follows:

1 1 2 NaN 3 0 4 dtype: object 

Problem:
The value at index 2 changed from None to NaN.

Expected Behaviour:
The value at index 2 should never be touched when using fillna and specifying the indices you want to alter.