Cast to subclass in the dmd compiler

[unDEFER via Digitalmars-d-learn]

Hello!
I'm hacking dmd compiler and trying to look on members array just 
after parse module.

for(uint i = 0; i < members.dim; i++)
{
     Dsymbol m = (*members)[i];

// It is good, but further:

    Import imp = cast(Import) m;
    if (imp !is null)
    {
        printf("++++ import %s.%s\n", imp.packages.toChars(), 
imp.id.toChars());
    }
// ...
}

For really imports casting doing well. But for not imports it 
again casts, imp not is null and  the compiler crashes.
What I'm doing wrong?