#Invalid Operation error with simple Index formula.

I am getting an #Invalid Operation error with a really simple index formula.

=INDEX(Contract:Contract, 2)

This formula is in a Sheet Summary field. The summary field is text/number and the column referenced is a text/number column with a column formula that produces numbers. The column has the Currency formatting on, but I have tried turning it off - just in case. The [Contract] column is a real column, there is definitely more than 1 row, so 2 as a row index should work.

I have tried the same Index formula with a different column in the sheet and had it work fine.

I have this same formula (in a more complex form) on many other sheets that are (like this one) based off the same template and all work fine.

For some reason, on this one sheet, it just gives me #Invalid Argument error. What am I missing?