Avoid installing optional yarn folders

In my file yarn.lock

I only have one momentjs link:

pikaday@^1.6.0:
  version "1.6.1"
  resolved "https://registry.yarnpkg.com/pikaday/-/pikaday-1.6.1.tgz#b91bcb9b8539cedd8d6d08e4e7465e12095671b0"
  optionalDependencies:
    moment "2.x"

      

        
        
        
      

    

I delete all folders node_modules

and yarn.lock

and run yarn --ignore-optional

, but the new yarn.lock

generated contains the same lines and installs moment

in my node_modules, so it is included in my bundle.

I am running a yarn version 0.27.5

.

How can I avoid installing this fingerprint marked as optional?