Setting index document on S3 website with custom domain

I'm trying to set an index document for my S3 hosted website. I've

1. Enabled website hosting on the bucket
2. Set the index document to "index.html"
3. Uploaded an index.html file
4. Made the index.html file public.

The index document works when navigating to the root of the bucket using the raw AWS url, but does not work when using a custom domain:

http://static.appgrinders.com.s3-website-us-east-1.amazonaws.com/index.html (works)
http://static.appgrinders.com.s3-website-us-east-1.amazonaws.com (works)
http://static.appgrinders.com/index.html (works)
http://static.appgrinders.com (Access denied - 403 error)