yarn install fails on empty string license with no warning

[FLGMwt]

Do you want to request a feature or report a bug?
bug

What is the current behavior?
Given a package.json with license: "", yarn fails with:

error TypeError: [email protected]: Cannot read property 'toUpperCase' of undefined
    at Array.<anonymous> (C:\Program Files (x86)\Yarn\node_modules\spdx-correct\index.js:82:23)
    at validTransformation (C:\Program Files (x86)\Yarn\node_modules\spdx-correct\index.js:173:36)
    at module.exports (C:\Program Files (x86)\Yarn\node_modules\spdx-correct\index.js:215:21)
    at module.exports (C:\Program Files (x86)\Yarn\node_modules\validate-npm-package-license\index.js:60:23)
    at isValidLicense (C:\Program Files (x86)\Yarn\lib\util\normalize-manifest\util.js:16:10)
    at exports.default (C:\Program Files (x86)\Yarn\lib\util\normalize-manifest\validate.js:53:55)
    at C:\Program Files (x86)\Yarn\lib\util\normalize-manifest\index.js:50:30
//...

If the current behavior is a bug, please provide the steps to reproduce.
Create a package.json with the license value ""

What is the expected behavior?
If a license key is present, but with an invalid value, output should be

warning [email protected]: License should be a valid SPDX license expression

As it is with license values such as "ASDFASDFqwertyqwerty"

Please mention your node.js, yarn and operating system version.
node v6.2.1
yarn 0.15.1
Windows 10