Renovate is not looking for Pipenv updates

I have the following configuration for a project, but so far Renovate is not opening merge requests updating the Pipfile:

{
  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
  "extends": [
    "config:base"
  ],
  "pipenv": {
    "enabled": true
  }
}

Wonder if it's a configuration thing or an upstream issue.