Estou tentando obter uma porcentagem do itemid disponível em uma determinada área. Usando minha consulta, recebo um erroORA-00937: not a single-group group function

Todos os detalhes:

Eu tenho essas duas tabelas:

ALLITEMS
---------------
ItemId  | Areas
---------------
1       | EAST
2       | EAST
3       | SOUTH
4       | WEST

CURRENTITEMS
---------------
ItemId
---------------
1
2
3

e deseja este resultado:

---------------
Areas| Percentage
---------------
EAST   | 50  --because ItemId 1 and 2 are in currentitems, so 2 items divided by the total 4 in allitems = .5
SOUTH  | 25   --because there is 1 item in currentitems table that are in area SOUTH (so 1/4=.25)
WEST   | 0  --because there are no items in currentitems that are in area WEST

O DDL:

drop table allitems;
drop table currentitems;

Create Table Allitems(ItemId Int,areas Varchar2(20));
Create Table Currentitems(ItemId Int);
Insert Into Allitems(Itemid,Areas) Values(1,'east');
Insert Into Allitems(ItemId,areas) Values(2,'east');
insert into allitems(ItemId,areas) values(3,'south');
insert into allitems(ItemId,areas) values(4,'east');

Insert Into Currentitems(ItemId) Values(1);
Insert Into Currentitems(ItemId) Values(2);
Insert Into Currentitems(ItemId) Values(3);

Minha consulta:

Select  
areas,
(
Select 
Count(Currentitems.ItemId)*100 / (Select Count(ItemId) From allitems inner_allitems Where inner_allitems.areas = outer_allitems.areas )
From 
Allitems Inner_Allitems Left Join Currentitems On (Currentitems.Itemid = Inner_Allitems.Itemid) 
Where inner_allitems.areas = outer_allitems.areas
***group by inner_allitems.areas***
***it worked by adding the above group by***
) "Percentage Result"
From 
allitems outer_allitems
Group By 
areas

O erro:

Error at Command Line:81 Column:41 (which is the part `(Select Count(ItemId) From allitems inner_allitems Where inner_allitems.areas = outer_allitems.areas )`)
Error report:
SQL Error: ORA-00937: not a single-group group function

Quando executo exatamente a mesma consulta no SQL Server, ele funciona bem. Como faço para corrigir isso no Oracle?