Convert of string to array problem

Have a string like '0.1;0.2;null;0.3' which I would like to convert
into a double precision[] array.

Trying:

select cast (string_to_array('0.1;0.2;null;0.3', ';') as float8[])

gives me an error: invalid input syntax for type double precision: "null".

Can anybody help me?

-Bård