fix:

lib/statum.rb

@@ -6,6 +6,7 @@
 module Statum
   UnknownEventError    = Class.new(ArgumentError)
   ErrorTransitionError = Class.new(StandardError)
+  ExistingMachineError = Class.new(ArgumentError)
 
   STATE_MACHINES_VARIABLE = '@__statum_machines'.freeze
 
@@ -31,7 +32,7 @@ def state_machines
 
     def add_machine(machine)
       if state_machines.any? { |m| m.name == machine.name }
-        raise "State machine for #{machine.name} already exists"
+        raise ExistingMachineError, "State machine for #{machine.name} already exists"
       end
       instance_variable_set(STATE_MACHINES_VARIABLE, state_machines + [machine])
     end

spec/multimachine_spec.rb

@@ -18,6 +18,17 @@ class MultiCar
   end
 end
 
+class ExistsCar
+  include Statum
+
+  attr_accessor :state, :engine
+
+  statum :state do
+    state :riding
+    state :idle
+  end
+end
+
 RSpec.describe "Statum multimachine" do
   let(:car) { MultiCar.new }
 
@@ -40,4 +51,9 @@ class MultiCar
     expect(car.state).to eq(:riding)
     expect(car.engine).to eq(:started)
   end
+
+  it "raises error when defining exists machine" do
+    expect { ExistsCar.instance_eval { statum :state } }
+      .to raise_error(Statum::ExistingMachineError)
+  end
 end