How to build an AWS ami using packer?

I have an existing AWS EC2 instance. I launched it with AMI ami-9a562df2. Then I installed some extra software on it. Now I want to create a new AMI of that instance (with the software I installed). I'm using packer to do it.

But I can't understand the error messages that it produces. Here is my JSON file (basically lifted from the packer website):

{
  "variables": {
    "aws_access_key": "",
    "aws_secret_key": ""
  },
  "builders": [{
    "type": "amazon-ebs",
    "access_key": "{{user `aws_access_key`}}",
    "secret_key": "{{user `aws_secret_key`}}",
    "region": "us-east-1",
    "source_ami": "ami-9a562df2",
    "instance_type": "t2.micro",
    "ssh_username": "ubuntu",
    "ami_name": "packer-example {{timestamp}}"
  }]
}

Here is what happens when I run that JSON file:

    $ ./packer build -var 'aws_access_key=MY_ACCESS_KEY' -var 'aws_secret_key=MY_SECRET_KEY' example.json
amazon-ebs output will be in this color.

==> amazon-ebs: Inspecting the source AMI...
==> amazon-ebs: Creating temporary keypair: packer 9999999-8888-7777-7777-666666666666
==> amazon-ebs: Creating temporary security group for this instance...
==> amazon-ebs: Authorizing SSH access on the temporary security group...
==> amazon-ebs: Launching a source AWS instance...
==> amazon-ebs: Error launching source instance: The specified instance type can only be used in a VPC. A subnet ID or network interface ID is required to carry out the request. (VPCResourceNotSpecified)
==> amazon-ebs: Deleting temporary security group...
==> amazon-ebs: Deleting temporary keypair...
Build 'amazon-ebs' errored: Error launching source instance: The specified instance type can only be used in a VPC. A subnet ID or network interface ID is required to carry out the request. (VPCResourceNotSpecified)

==> Some builds didn't complete successfully and had errors:
--> amazon-ebs: Error launching source instance: The specified instance type can only be used in a VPC. A subnet ID or network interface ID is required to carry out the request. (VPCResourceNotSpecified)

How can I avoid getting the error Error launching source instance: The specified instance type can only be used in a VPC. A subnet ID or network interface ID is required to carry out the request. (VPCResourceNotSpecified)?

I'm sure this seems elementary to some. But I don't know why it is asking me for a VPC and Subnet ID. What am I missing about this operation?