diff --git a/CHANGELOG.md b/CHANGELOG.md
index ed92ff3..bc99ff8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,7 +6,7 @@ This project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html
 ## Unreleased
 
 ### Compatible changes
--
+- Fix error "undefined method `[]' for nil" due to missing code line in previous version
 
 ### Breaking changes
 -
diff --git a/lib/rails_state_machine/model.rb b/lib/rails_state_machine/model.rb
index a397f09..f64ae9e 100644
--- a/lib/rails_state_machine/model.rb
+++ b/lib/rails_state_machine/model.rb
@@ -36,6 +36,7 @@ def state_events(state_attribute = DEFAULT_STATE_ATTRIBUTE)
     private
 
     def state_machine_state_manager(state_attribute)
+      @state_machine_state_managers ||= {}
       @state_machine_state_managers[state_attribute] ||= StateManager.new(self, state_machine(state_attribute), state_attribute)
     end