Union All of two tables based on month of a date in Oracle

I have two tables table_a and table_b as below. column invdate is of date datatype and column amount is of number datatype.

table_a

invdate amount
20-01-2021 50
20-01-2021 100
20-02-2021 50
20-03-2021 50

table_b

invdate amount
01-01-2021 250
01-02-2021 300
01-03-2021 40
01-03-2021 50

I am doing a UNION ALL of both tables to get the sum of the amounts based on only month and year.

SELECT to_char(invdate, 'MM-YYYY') as "Date", sum(amount) as "Total" FROM ( SELECT to_char(invdate, 'MM-YYYY'), amount FROM table_a UNION ALL SELECT to_char(invdate, 'MM-YYYY'), amount FROM table_b ) GROUP BY to_char(invdate, 'MM-YYYY') ORDER BY to_char(invdate, 'MM-YYYY') asc; 

to get the final output like below

Date Total
01-2021 400
02-2021 350
03-2021 140

but it gives me the below error.

ORA-00904: "INVDATE": invalid identifier 00904. 00000 -  "%s: invalid identifier" *Cause:     *Action: Error at Line: 10 Column: 18 

What am I doing wrong here?