SQL & PL/SQL

Hi,
I'm trying find optimal solution for problem:

I have query:

SELECT
  col_2,
  SUM(col_1)
FROM table_1
GROUP BY
col_2;

in col_1 are possible NULL values, how to make SUM return NULL if any of values col_1 in SUM for specific col_2 is NULL.
I have some solution but its very heavy :

...
CASE
  WHEN SUM(col_1) = SUM(NVL(col_1,-1000))
  THEN SUM(col_1)
  ELSE NULL
END
...

Does anyone see some better solution?

P.S. analytical function are not very welcome :)
Thanks for help

--
Krzysiek Wojszko