There are two solutions to this issue depending on which version of wiredep you want to use.
If you want to use ‘^1.9.0’, make sure to remove the cwd property from your Gruntfile.js. This is a common issue if you are an angular-generator user which currently specifies a cwd property on the config for the wiredep task.
If you don’t mind using ‘1.8.0’, make sure to pin that version in your package.json. If you are including wiredep via grunt-wiredep, then you will have to add wiredep manually and pin it. In the case that you stick with ‘1.8.0’, leave the cwd property in the config for the task.