Fail to deploy mongo atlas

Hi guys, Before I raise this as a bug I thought I would ask here first in case I am doing something stupid, which wouldn’t be the first time.

Terraform version 0.12.20
mongodbatlas provider version 0.4.0

So trying to deploy to an Azure environment we attempt to create a mongo atlas cluster via terraform, but get the following error:

Error: error getting Project IP Whitelist information: whiteListEntry is invalid because must be set

The following entry validates ok (ip address redacted), but generates the above error at run time :

resource “mongodbatlas_project_ip_whitelist” “test” {
project_id = mongodbatlas_project.test[0].id

cidr_block = “”
comment = “Test whitelist (terraform managed)”

Any clues suggestions or pointers gladly received!