OP_SUBDOMAIN enviroment not having expected effect

[sjames-au]

I have OP_SUBDOMAIN set to 'the1psubdomain', e.g.
export OP_SUBDOMAIN="the1psubdomain"

The dcoumentation indicates this should be all that is required for this provider to defer to using OP_SESSION_the1psubdomain.

However, the plan stage returns:
Error: email, password or secret_key is empty and environment variable OP_SESSION_my is not set

DefaultFunc doco at https://www.terraform.io/docs/extend/schemas/schema-behaviors.html#defaultfunc states that it can not be used if a Default is specified but provides a reasonable example approach to return a default if required.