Failed route association

{
  "variable": {
    "region": {
      "default": "us-east-1"
    },
    "key_name": {},
    "vpc_cidr": {
      "default": "11.0.0.0/16"
    },
    "zones": {
      "default": 2
    },
    "zone_names": {
      "default": {
        "0": "a",
        "1": "b"
      }
    },
    "public_cidrs": {
      "default": {
        "0": "11.0.253.0/24",
        "1": "11.0.254.0/24"
      }
    },
    "private_cidrs": {
      "default": {
        "0": "11.0.1.0/24",
        "1": "11.0.2.0/24"
      }
    }
  },
  "resource": {
    "aws_vpc": {
      "main": {
        "cidr_block": "${var.vpc_cidr}",
        "enable_dns_hostnames": true
      }
    },
    "aws_internet_gateway": {
      "main": {
        "vpc_id": "${aws_vpc.main.id}"
      }
    },
    "aws_subnet": {
      "public": {
        "count": "${var.zones}",
        "vpc_id": "${aws_vpc.main.id}",
        "cidr_block": "${lookup(var.public_cidrs, count.index)}",
        "availability_zone": "${var.region}${lookup(var.zone_names, count.index)}",
        "map_public_ip_on_launch": true,
        "depends_on": [
          "aws_internet_gateway.main"
        ]
      },
      "private": {
        "count": "${var.zones}",
        "vpc_id": "${aws_vpc.main.id}",
        "cidr_block": "${lookup(var.private_cidrs, count.index)}",
        "availability_zone": "${var.region}${lookup(var.zone_names, count.index)}"
      }
    },
    "aws_route_table": {
      "public": {
        "vpc_id": "${aws_vpc.main.id}",
        "route": {
          "cidr_block": "0.0.0.0/0",
          "gateway_id": "${aws_internet_gateway.main.id}"
        }
      },
      "private": {
        "vpc_id": "${aws_vpc.main.id}",
        "route": {
          "cidr_block": "0.0.0.0/0",
          "instance_id": "${aws_instance.bastion_nat.id}"
        }
      }
    },
    "aws_route_table_association": {
      "public": {
        "count": "${var.zones}",
        "subnet_id": "${element(aws_subnet.public.*.id, count.index)}",
        "route_table_id": "${aws_route_table.public.id}"
      },
      "private": {
        "count": "${var.zones}",
        "subnet_id": "${element(aws_subnet.public.*.id, count.index)}",
        "route_table_id": "${aws_route_table.private.id}"
      }
    },
    "aws_instance": {
      "bastion_nat": {
        "ami": "ami-303b1458",
        "instance_type": "t2.micro",
        "subnet_id": "${aws_subnet.public.0.id}",
        "source_dest_check": false,
        "key_name": "${var.key_name}"
      }
    }
  }
}%