It must be friday, my brain has locked up already...

I want to do a query on a table that sums a column that may have null values. So I type in:

select field1, nvl(sum(field2),0)
from table
group by field1

ERROR:
ORA-24347: Warning of a NULL column in an aggregate function

Interestingly enough, the same query without the NVL function returns results when I run it in a TOAD window.

I'm missing something obvious here... what is it?