Getting Daily Average Of Sales From Monthly Report?

I have this small project, I have this report that have the total of order along with the date of the order

SELECT sf.ORDER_QNT, dd.ACTUAL_DATE, dd.MONTH_NUMBER
FROM sales_fact sf,
date_dim dd
WHERE dd.date_id = sf.date_id
AND dd.MONTH_NUMBER = 1;

ORDER_QNT ACTUAL_DATE MONTH_NUMBER
1100 05/01/13 1
100 05/01/13 1
140 06/01/13 1
110 07/01/13 1
200 08/01/13 1
500 08/01/13 1
230 08/01/13 1
500 08/01/13 1
200 08/01/13 1
53 15/01/13 1
53 22/01/13 1

Now, I want to get the average for that month (average per day).

SELECT sum(sf.ORDER_QNT)/31 as AVGPERDAY
FROM sales_fact sf,
date_dim dd
WHERE dd.date_id = sf.date_id
AND dd.MONTH_NUMBER = 1;

AVGPERDAY MONTH_NUMBER
---------- ------------
113.785714 1

but instead putting 31, I'd like to pull the totaldays from the actual_date using the Extract function so I try this

SELECT sum(sf.ORDER_QNT)/EXTRACT(DAY FROM LAST_DAY(to_date('05/01/13','dd/mm/rr'))) as AVGPERDAY,
dd.month_number
FROM sales_fact sf,
date_dim dd
WHERE dd.date_id = sf.date_id
AND dd.month_number = 1
GROUP BY dd.month_number;

AVGPERDAY MONTH_NUMBER
---------- ------------
113.785714 1

The result is nice, but now when I change the date with the dd.actual_date it gives error

SELECT sum(sf.ORDER_QNT)/EXTRACT(DAY FROM LAST_DAY(dd.actual_date)) as AVGPERDAY,
dd.month_number
FROM sales_fact sf,
date_dim dd
WHERE dd.date_id = sf.date_id
AND dd.month_number = 1
GROUP BY dd.month_number;
Error at Command Line : 1 Column : 53

Error report -
SQL Error: ORA-00979: not a GROUP BY expression
00979. 00000 - "not a GROUP BY expression"