# Buggy code
def create_subnet(conn, module, vpc_id, cidr, az, check_mode):
    try:
        new_subnet = get_subnet_info(
            conn.create_subnet(VpcId=vpc_id, CidrBlock=cidr, AvailabilityZone=az)
        )
        # Sometimes AWS takes its time to create a subnet and so using
        # new subnets's id to do things like create tags results in
        # exception.  boto doesn't seem to refresh 'state' of the newly
        # created subnet, i.e.: it's always 'pending'.
        subnet = False
        while subnet is False:
            subnet = subnet_exists(conn, new_subnet["id"])
            time.sleep(0.1)
    except botocore.exceptions.ClientError as e:
        if e.response["Error"]["Code"] == "DryRunOperation":
            subnet = None
        else:
            module.fail_json(
                msg=e.message,
                exception=traceback.format_exc(),
                **camel_dict_to_snake_dict(e.response),
            )

    return subnet


# Traceback (botocore.exceptions.ParamValidationError)
The full traceback is:
Traceback (most recent call last):
File "/tmp/ansible_jmC5F8/ansible_module_ec2_vpc_subnet.py", line 274, in <module>
main()
File "/tmp/ansible_jmC5F8/ansible_module_ec2_vpc_subnet.py", line 262, in main
check_mode=module.check_mode)
File "/tmp/ansible_jmC5F8/ansible_module_ec2_vpc_subnet.py", line 189, in ensure_subnet_present
subnet = create_subnet(conn, module, vpc_id, cidr, az, check_mode)
File "/tmp/ansible_jmC5F8/ansible_module_ec2_vpc_subnet.py", line 127, in create_subnet
new_subnet = get_subnet_info(conn.create_subnet(VpcId=vpc_id, CidrBlock=cidr, AvailabilityZone=az))
File "/usr/local/lib/python2.7/dist-packages/botocore/client.py", line 312, in _api_call
return self._make_api_call(operation_name, kwargs)
File "/usr/local/lib/python2.7/dist-packages/botocore/client.py", line 575, in _make_api_call
api_params, operation_model, context=request_context)
File "/usr/local/lib/python2.7/dist-packages/botocore/client.py", line 630, in _convert_to_request_dict
api_params, operation_model)
File "/usr/local/lib/python2.7/dist-packages/botocore/validate.py", line 291, in serialize_to_request
raise ParamValidationError(report=report.generate_report())
botocore.exceptions.ParamValidationError: Parameter validation failed:
Invalid type for parameter AvailabilityZone, value: None, type: <type 'NoneType'>, valid types: <type 'basestring'>

failed: [localhost] (item=10.10.0.0/24) => {
"changed": false,
"failed": true,
"item": "10.10.0.0/24",
"module_stderr": "Traceback (most recent call last):\n  File \"/tmp/ansible_jmC5F8/ansible_module_ec2_vpc_subnet.py\", line 274, in <module>\n    main()\n  File \"/tmp/ansible_jmC5F8/ansible_module_ec2_vpc_subnet.py\", line 262, in main\n    check_mode=module.check_mode)\n  File \"/tmp/ansible_jmC5F8/ansible_module_ec2_vpc_subnet.py\", line 189, in ensure_subnet_present\n    subnet = create_subnet(conn, module, vpc_id, cidr, az, check_mode)\n  File \"/tmp/ansible_jmC5F8/ansible_module_ec2_vpc_subnet.py\", line 127, in create_subnet\n    new_subnet = get_subnet_info(conn.create_subnet(VpcId=vpc_id, CidrBlock=cidr, AvailabilityZone=az))\n  File \"/usr/local/lib/python2.7/dist-packages/botocore/client.py\", line 312, in _api_call\n    return self._make_api_call(operation_name, kwargs)\n  File \"/usr/local/lib/python2.7/dist-packages/botocore/client.py\", line 575, in _make_api_call\n    api_params, operation_model, context=request_context)\n  File \"/usr/local/lib/python2.7/dist-packages/botocore/client.py\", line 630, in _convert_to_request_dict\n    api_params, operation_model)\n  File \"/usr/local/lib/python2.7/dist-packages/botocore/validate.py\", line 291, in serialize_to_request\n    raise ParamValidationError(report=report.generate_report())\nbotocore.exceptions.ParamValidationError: Parameter validation failed:\nInvalid type for parameter AvailabilityZone, value: None, type: <type 'NoneType'>, valid types: <type 'basestring'>\n",
"module_stdout": "",
"msg": "MODULE FAILURE",
"rc": 0
}

# Fixed code
def create_subnet(conn, module, vpc_id, cidr, az, check_mode):
    try:
        # Specifying the availability zone is optional.
        if not az:
            az = ""
        new_subnet = get_subnet_info(
            conn.create_subnet(VpcId=vpc_id, CidrBlock=cidr, AvailabilityZone=az)
        )
        # Sometimes AWS takes its time to create a subnet and so using
        # new subnets's id to do things like create tags results in
        # exception.  boto doesn't seem to refresh 'state' of the newly
        # created subnet, i.e.: it's always 'pending'.
        subnet = False
        while subnet is False:
            subnet = subnet_exists(conn, new_subnet["id"])
            time.sleep(0.1)
    except botocore.exceptions.ClientError as e:
        if e.response["Error"]["Code"] == "DryRunOperation":
            subnet = None
        else:
            module.fail_json(
                msg=e.message,
                exception=traceback.format_exc(),
                **camel_dict_to_snake_dict(e.response),
            )

    return subnet