diff --git a/railties/lib/rails/generators/base.rb b/railties/lib/rails/generators/base.rb
index 9624c35c0b..27e8d8483d 100644
--- a/railties/lib/rails/generators/base.rb
+++ b/railties/lib/rails/generators/base.rb
@@ -1,4 +1,12 @@
-require 'thor/group'
+begin
+  require 'thor/group'
+rescue LoadError
+  puts "Thor is not available.\nIf you ran this command from a git checkout " \
+       "of Rails, please make sure thor is installed,\nand run this command " \
+       "as `ruby -rubygems /path/to/rails myapp --dev`"
+  exit
+end
+
 require 'rails/generators/actions'
 
 module Rails