vr_ad with reserved bits but writable

Hi 'e' folks,

I'm working in the verification of a SOC using vr_ad last version:

reg def BLABLA_0 BLA_REGS 0x01 {
    reg_fld unused  : uint(bits:28) : R: 0:cov;
    reg_fld foo : uint(bits:4):RW:0:cov;
    set_static_info() is {
       static_info.compare_mask = 0xFFFFFFFF;  // compare up to 32 bits
       static_info.description = "Control Reg";
 };

In my test, I perform write -read access on my register: the thing I do not understand , Why the write value chosen by bfm unit does not take care of the fact that unused is R (read only).

Actually, the write value is 0x5e006a9 and of course I collect a dut error due to a mismatch because my unused is R on a spec point of view but writable in DUT.

Have you some hint to constrain the ability to :
perform write with appropriate width on such register (R reg_field and RW reg fields)
and perform read with appropriate width when you have W reg fields and RW reg_fields

Cheers 

Fred