AWS EKS – Troubleshooting kubectl Authentication Issues

I cannot get kubectl to authenticate with the EKS Kubernetes instance my coworker created. I've followed the documentation: the AWS CLI can run aws eks commands (I'm an AWS Full Administrator), and the heptio authenticatior is in my path and can generate tokens.

When I run kubectl I get this error:

$ kubectl version
Client Version: version.Info{Major:"1", Minor:"10", GitVersion:"v1.10.4", 
GitCommit:"5ca598b4ba5abb89bb773071ce452e33fb66339d", GitTreeState:"clean", 
BuildDate:"2018-06-06T15:22:13Z", GoVersion:"go1.9.6", Compiler:"gc", 
Platform:"darwin/amd64"}
error: You must be logged in to the server (the server has asked for the client
to provide credentials)

Here's my ~/.kube/config file. It's the exact kubeconfig my coworker can successfully use.

apiVersion: v1
clusters:
- cluster:
    server: https://myinstance.sk1.us-east-1.eks.amazonaws.com
    certificate-authority-data: base64_cert                                                                                                                                                                                                   name: kubernetes                                                                                                                                                                                                                          contexts:                                                                                                                                                                                                                                   - context:                                                                                                                                                                                                                                      cluster: kubernetes                                                                                                                                                                                                                         user: aws                                                                                                                                                                                                                                 name: aws
current-context: aws
kind: Config
preferences: {}
users:
- name: aws
  user:
    exec:
      apiVersion: client.authentication.k8s.io/v1alpha1
      command: heptio-authenticator-aws
      args:
        - "token"
        - "-i"
        - "dev-qa"
        # - "-r"
        # - "<role-arn>"