Index: db/migrate/091_change_changesets_revision_to_string.rb
===================================================================
--- db/migrate/091_change_changesets_revision_to_string.rb	(revision 11125)
+++ db/migrate/091_change_changesets_revision_to_string.rb	(working copy)
@@ -1,6 +1,8 @@
 class ChangeChangesetsRevisionToString < ActiveRecord::Migration
   def self.up
-    remove_index  :changesets, :name => :changesets_repos_rev
+    if index_exists? :changesets, :changesets_repos_rev
+    	remove_index  :changesets, :name => :changesets_repos_rev
+    end
     change_column :changesets, :revision, :string, :null => false
     add_index :changesets, [:repository_id, :revision], :unique => true, :name => :changesets_repos_rev
   end