Deploying .NET with Beanstalk: must include '.runtimeconfig.json'

I am trying to deploy a .NET application on Linux using the .NET core on Linux deployment option in Elastic Beanstalk. Due to account permissions I can't use the plugin for VS, so I'm trying to upload a ZIP of the published code. Everything seems to be in place, and I can deploy the sample application code with no problems. But when I try to upload my own code, I get the following error, even though the Website.runtimeconfig.json file is present in the uploaded .zip file:

Instance deployment: Your source bundle has a single .NET Core application. You must include a file with a '.runtimeconfig.json' suffix. The deployment failed.

Any idea what I might be missing?