Print a warning when a gem can't be cached due to a missing source

1 files changed, 5 insertions, 1 deletions

diff --git a/lib/bundler/source/rubygems.rb b/lib/bundler/source/rubygems.rb
index d15362b9aa..a9b10f5936 100644
--- a/lib/bundler/source/rubygems.rb
+++ b/lib/bundler/source/rubygems.rb
@@ -189,7 +189,11 @@ module Bundler
         cached_path = cached_path(spec)
         if cached_path.nil?
           remote_spec = remote_specs.search(spec).first
-          cached_path = fetch_gem(remote_spec)
+          if remote_spec
+            cached_path = fetch_gem(remote_spec)
+          else
+            Bundler.ui.warn "#{spec.full_name} is built in to Ruby, and can't be cached because your Gemfile doesn't have any sources that contain it."
+          end
         end
         cached_path
       end