'Group by' combined with 'where' not working

[hvt]

Suppose I have the following serie:

a | b | c
---------
x | y | 10
x | y | 20
y | z | 30
x | z | 40

And now issue the following query:

select sum(c) from test group by b

Then I get the expected:

time | sum | b
--------------
tttt | 30  | y
tttt | 70  | z

Now I issue:

select sum(c) from test group by b where a == 'x'
or:
select sum(c) from test where a == 'x' group by b

I would expect:

time | sum | b
--------------
tttt | 30  | y
tttt | 40  | z

I do however get the following query error:

Invalid column name b