Oddities with NULL and GROUP BY

Maybe there is something I don't know about how GROUP BY should
work, but if I have a table like:
a,b,c
1,1,1
1,1,2
1,1,3
1,2,1
1,3,1

And I say SELECT a,b,sum(c) FROm .. GROUP BY a,b I get
1,1,6
1,2,1
1,3,1

So whenever a or b changes we get a new summed row, well if I have rows
where a or b are null, this doesn't happen, infact I seem to get all
those rows individually... Like if:
1,1,1
1,1,3
1,NULL,10
1,NULL,20
1,2,3

I get:
1,1,4
1,NULL,10
1,NULL,20
1,2,3

Shouldn't I get 1,NULL,30? Ie shouldn't NULL be treated like any other
value? Or is there some bit of information I'm missing? I can set
everything from NULL to 0 if need be, but I'd rather not...

David Secret
MIS Director
Kearney Development Co., Inc.