How to override a role default dictionary variable value from command line

[Ajay B]

I have in a role(A) a dictionary variable defined  under defaults as follows :
ceph_disks:
  /dev/vdb: "/es1"

This role A is being included in another role B thru dependencies in meta file. When this role B is executed and command line argument passed thru extra vars as follows:
--extra-vars="env=thor storage_type=ceph  {'ceph_disks':{'/dev/vdb': 'hdfs'}}"

This value is not being overridden for default value of es1 defined earlier.

How to override this default value ?

Thanks