[Python] python pandas "cannot set a row with mismatched columns" error

I am facing an issue while adding a new row to the data set.

Here is the example DataFrame.

column_names = ['A','B','C']
items = [['a1','b1','c1'],['a2','b2']]
newDF = pd.DataFrame(items,columns=column_names)
print(newDF)


output:

A B C
0 a1 b1 c1
1 a2 b2 None


Since c2 was missing, it was replaced with None. This is fine and as expected.

Now if i continue to add similar rows to this existing DataFrame, like this:

newDF.loc[len(newDF)] = ['a3','b3']


I get the error "cannot set a row with mismatched columns".

How can I add this additional row, so that it will automatically take care of missing c3 with None or NaN?

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