=SUMPRODUCT(--($AI$2:$AI$400=$AM$4),--($E$2:$E$400="TWINSTATIA"),$X$2:$X$400)
returns a value error.

AI2:AI400 is a column with the month billed (i.e. Jan, Feb, March, etc...)
AM4 is the word "Dec" (the only month I want to include in the sum product)
E2:E400 is a column with the customer name (in this case, I want to only
total those with TWINSTATIA)
X2:X400 is a column with the total pay (each cell has a formula)

Any suggestions? This same formula works for previous months. I have
rechecked to make sure the month is spelled right & the right cells are
referenced.

Thank you.