Adding custom serverless.yml properties breaks execution

**Actual Behaviour**

Per the suggestions in the README, I've added the following section to my `serverless.yml` configuration:

```yaml
custom:
dynamodb:
stages:
- dev
```

(For clarity, my stage is defined as `dev` at a provider level.)

What I'm now seeing is that all commands to `sls dynamodb ` (with the env variable `SLS_DEBUG=*`) follow the same trajectory to failure. As an example:

```bash
$ > sls dynamodb start --migrate
Serverless: Load command config
# Other commands truncated
Serverless: Load command dynamodb
Serverless: Load command dynamodb:migrate
Serverless: Load command dynamodb:seed
Serverless: Load command dynamodb:start
Serverless: Load command dynamodb:noStart
Serverless: Load command dynamodb:remove
Serverless: Load command dynamodb:install
Serverless: Invoke dynamodb:start
Serverless: Warning: The command you entered did not catch on any hooks
```

**Expected Behaviour**

When I remove the custom `yml` configuration, then it behaves as expected. After the `invoke` logline, the command runs successfully.

Start with the serverless.yml configuration shown in the report and trace how the custom.dynamodb.stages setting is read during the dynamodb:start command. Compare execution with and without that setting, following the logged dynamodb:start invocation and command hooks; done means sls dynamodb start --migrate runs successfully while the custom configuration remains present.

Written by the indexing model from the issue text.