new to D2.0 , invariant problem

[MikeRJ]

For some reason i think DM 1.0 is slowly dieing as i see tons of new features 
in D 2.0's phobos not implemented in D 1.0  so im trying to migrate to D2.0

my 1st problem is when i compile code:

        bool open(char[] fn, char[] mode="rb")
        {
                filename=fn.dup;
                file = fopen(toStringz(filename), toStringz(mode));
                if (!file) return false;
                return true;
        }

i receive error:

>> cannot implicitly convert expression ("rb") of type invariant(char[2u]) to 
>> char[]|
can anyone help here. thanks in advance.