Sum the digits of a single cell down to one number

Hello all!

I saw that this function exists for adding all the digits in a cell (for example, A1) into a single number in B1.

=A1 - FLOOR(A1 - 1, 9)

For example, if I typed the above code into B1, and if A1 was 33, it would take 3+3=6, giving me 6 as the number that shows up in B1. Another example would be for 93, it would take 9+3=12, then 1+2=3, giving me 3 as my end result in B1.

The problem I run into is when the number in A1 is 0. The same code is put into B1, but B1 shows up as 9, not 0. I tried to fiddle around with making an exception to the formula, but I can't seem to figure it out. Is there a way to keep that formula in there, but have 0 show up if the number in A1 is 0? Thanks in advance for your help!