WEBSITES_INTERNAL_FREESKU_SITE not allowed

[mcollier]

I'm attempting to add a new environment variable, FOO_BAR, with a value of TEST. When I try to save/apply this change, I get the following error in the Azure Portal.

Failed to update app settings:
AppSettings is invalid.  {"Error":{"Code":"InvalidAppSettings","Message":"AppSetting with name 'WEBSITES_INTERNAL_FREESKU_SITE' is not allowed.","ExtendedData":null,"Details":null}}

I'm not trying to set WEBSITES_INTERNAL_FREESKU_SITE. I'm not even using a free site, but am on a Standard SKU.

Using the Azure CLI to edit the environment variable also fails with a "Bad Request" error.

Any suggestions on how to work around this issue?