Constructing pandas DataFrame from values in variables gives ValueError If using all scalar values you must pass an index

Question

If  I have two variables as follows:

a = 2
b = 3

And I want to construct a DataFrame from this:

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

But when I try to do that, it gives an error:

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

To solve this I tried :

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

But it gave me the same error. Can someone help me with this?