Provider "core" not available for installation

I am trying to set up a terraform module that references another through a remote_state, and uses modules itself. However, when I try to initialize the module, I get

dumontj ~/Projects/azure/terraform/DEV-environment/actual_project_1 $ terraform init
Initializing modules...

Initializing the backend...

Initializing provider plugins...
- Checking for available provider plugins...

Provider "core" not available for installation.

A provider named "core" could not be found in the Terraform Registry.

This may result from mistyping the provider name, or the given provider may
be a third-party provider that cannot be installed automatically.

In the latter case, the plugin must be installed manually by locating and
downloading a suitable distribution package and placing the plugin's executable
file in the following directory:

Terraform detects necessary plugins by inspecting the configuration and state.
To view the provider versions requested by each module, run
"terraform providers".

Running terraform providers, as suggested, gives:

$ terraform providers
├── provider.azurerm =2.8.0
├── provider.terraform
└── module.client_project
    ├── provider.azurerm (inherited)
    ├── provider.core
    └── module.subnet_module_test
        └── provider.azurerm (inherited)

Where does provider.core come from? Google didn’t turn up anything, and I don’t have any resources called “core” anything in my module. Debug output only says that

2020/05/25 08:37:02 [DEBUG] plugin requirements: "core"=""

but doesn’t say how it determined that this plugin was needed.

How can I fix this?