New table with sums dependent on month not working

Hi, I have data with
* Customer ID

* Item ID

* Month

* Sales

In the months from July through December

I want to create a new table with 
* Customer ID

* Item ID

* Sum of sales from July to November

* Sales from December

Customer/Item Sales =
SUMMARIZE (
    Append1,
    Append1[Customer ID],
    Append1[Item ID],
    "July-November Sales", CALCULATE ( SUM ( Append1[Sales] ), Append1[Month] < MONTH ( 12 ) ),
    "December Sales", CALCULATE ( SUM ( Append1[Sales] ), Append1[Month] = MONTH ( 12 ) )
)

This code returns Customer ID and Item ID, but the other two columns I wanted to create show as blanks. What am I doing wrong?

Thanks