Cannot read property 'split' of undefined

[sysboss]

Getting the following error:

$ sls raml
 
  Type Error ---------------------------------------------
 
  Cannot read property 'split' of undefined

Reproduce:

1. Install npm install --save raml-serverless
2. Add raml-serverless to your serverless.yml

$ git diff serverless.yml
 plugins:
   - serverless-python-requirements
+  - raml-serverless

3. sls raml > docs.raml

Node version: v6.10.1 Serverless version 1.18.0