Constructing DataFrame from values in variables yields "ValueError: If using all scalar values, you must pass an index"

I have two variables as follows.

a = 2
b = 3

I want to construct a DataFrame from this:

df2 = pd.DataFrame({'A':a, 'B':b})

This generates an error:

ValueError: If using all scalar values, you must pass an index

I tried this also:

df2 = (pd.DataFrame({'a':a, 'b':b})).reset_index()

This gives the same error message. How do I do what I want?