`dep ensure` fails

[yuku]

When I tried to add firebase.google.com/go using dep, following error occurred:

$ dep ensure -add firebase.google.com/go
The following errors occurred while deducing packages:
  * "code.google.com/p/freetype-go/freetype/raster": unable to deduce repository and source type for "code.google.com/p/freetype-go/freetype/raster": unable to read metadata: go-import metadata not found

validateParams: could not deduce external imports' project roots

I'm not sure the issue relates to this repository, http://code.google.com/p/freetype-go/freetype/raster redirects to this repository.

environment

$ dep version
dep:
 version     : v0.4.1
 build date  : 2018-01-27
 git hash    : 37d9ea0
 go version  : go1.9.3
 go compiler : gc
 platform    : darwin/amd64

[yuku]

My project root directory had go_appengine SDK and removing it fixed the issue.