rake db:migrate returns 2 ActiveAdmin warnings and a 'rake aborted' message

[oveits]

There are 2 ActiveAdmin warnings, a rake aborted message and a Graphviz failure (cause by erb gem)

Resource does not exist)
Warning: Ignoring invalid association :author on ActiveAdmin::Comment (polymorphic interface Author does not exist)
rake aborted!
Saving diagram failed!
Verify that Graphviz is installed and in your path, or use filetype=dot.

Although it says that rake was aborted, the migration is still successful. Therefore prio low.

Full log:

rake db:migrate RAILS_ENV=development
== 20160205121053 RemoveConfigs: migrating ====================================
-- drop_table(:configs)
   -> 0.0010s
== 20160205121053 RemoveConfigs: migrated (0.0011s) ===========================

Loading application environment...
Loading code in search of Active Record models...
Generating Entity-Relationship Diagram for 12 models...
Warning: Ignoring invalid association :resource on ActiveAdmin::Comment (polymorphic interface Resource does not exist)
Warning: Ignoring invalid association :author on ActiveAdmin::Comment (polymorphic interface Author does not exist)
rake aborted!
Saving diagram failed!
Verify that Graphviz is installed and in your path, or use filetype=dot.
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/rails_erd/diagram/graphviz.rb:207:in `rescue in block in <class:Graphviz>'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/rails_erd/diagram/graphviz.rb:198:in `block in <class:Graphviz>'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/rails_erd/diagram.rb:142:in `instance_eval'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/rails_erd/diagram.rb:142:in `save'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/rails_erd/diagram.rb:120:in `create'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/rails_erd/diagram.rb:74:in `create'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/rails_erd/tasks.rake:41:in `block (2 levels) in <top (required)>'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/tasks/auto_generate_diagram.rake:18:in `update_model'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/tasks/auto_generate_diagram.rake:3:in `block (2 levels) in <top (required)>'
/usr/local/rvm/gems/ruby-2.2.4/bin/ruby_executable_hooks:15:in `eval'
/usr/local/rvm/gems/ruby-2.2.4/bin/ruby_executable_hooks:15:in `<main>'
StandardError: GraphViz not installed or dot not in PATH. Install GraphViz or use the 'path' option
/usr/local/rvm/gems/ruby-2.2.4/gems/ruby-graphviz-1.2.2/lib/graphviz.rb:544:in `output'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/rails_erd/diagram/graphviz.rb:200:in `block in <class:Graphviz>'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/rails_erd/diagram.rb:142:in `instance_eval'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/rails_erd/diagram.rb:142:in `save'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/rails_erd/diagram.rb:120:in `create'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/rails_erd/diagram.rb:74:in `create'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/rails_erd/tasks.rake:41:in `block (2 levels) in <top (required)>'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/tasks/auto_generate_diagram.rake:18:in `update_model'
/usr/local/rvm/gems/ruby-2.2.4/gems/rails-erd-1.4.5/lib/tasks/auto_generate_diagram.rake:3:in `block (2 levels) in <top (required)>'
/usr/local/rvm/gems/ruby-2.2.4/bin/ruby_executable_hooks:15:in `eval'
/usr/local/rvm/gems/ruby-2.2.4/bin/ruby_executable_hooks:15:in `<main>'
Tasks: TOP => erd => erd:generate
(See full trace by running task with --trace)