Subnet AzureFirewallSubnet is in use and cannot be deleted

Hi

I am trying to deploy a Azure Firewall using ARM templates. The template works fine during first deploy and creates a subnet (named AzureFirewallSubnet as required) in a existing virtual network as well as a Azure Firewall with a public IP. When trying to deploy the template a second time expecting the deployment to pass through without changes I get following error:

"Subnet AzureFirewallSubnet is in use ...<firewall-name>/azureFirewallIpConfigurations/IpConf and cannot be deleted"

Here is the detailed error message from the arm template deployment:

"details": [
      {"code": "DeploymentFailed","message": "At least one resource deployment operation failed. Please list deployment operations for details. Please see https://aka.ms/arm-debug for usage details.","details": [
          {"code": "BadRequest","message": "{\r\n  \"error\": {\r\n    \"code\": \"InUseSubnetCannotBeDeleted\",\r\n    \"message\": \"Subnet AzureFirewallSubnet is in use by /subscriptions/<subscription-id>/resourceGroups/<resource-group-name>/providers/Microsoft.Network/azureFirewalls/<firewall-name>/azureFirewallIpConfigurations/IpConf and cannot be deleted.\",\r\n    \"details\": []\r\n  }\r\n}"
          }
        ]
      }
    ]

Any clues?

Mattias