diff --git a/lib/ruote/engine.rb b/lib/ruote/engine.rb
index df1151ab..260c1bd3 100644
--- a/lib/ruote/engine.rb
+++ b/lib/ruote/engine.rb
@@ -521,15 +521,13 @@ def load_definition (path)
     # containing the participant implementation. 'require' will load and eval
     # the ruby code only once, 'load' each time.
     #
-    def register_participant (regex, participant=nil, opts=nil, &block)
+    def register_participant (regex, participant=nil, opts={}, &block)
 
       if participant.is_a?(Hash)
         opts = participant
         participant = nil
       end
 
-      opts ||= {}
-
       pa = @context.plist.register(regex, participant, opts, block)
 
       @context.storage.put_msg('participant_registered', 'regex' => regex.to_s)
diff --git a/lib/ruote/svc/participant_list.rb b/lib/ruote/svc/participant_list.rb
index 08e32da9..18679c0b 100644
--- a/lib/ruote/svc/participant_list.rb
+++ b/lib/ruote/svc/participant_list.rb
@@ -47,6 +47,11 @@ def initialize (context)
     #
     def register (name, participant, options, block)
 
+      raise(
+        ArgumentError.new(
+          "can only accept strings (classnames) or classes as participant arg")
+      ) unless [ String, Class, NilClass ].include?(participant.class)
+
       klass = (participant || Ruote::BlockParticipant).to_s
 
       options = options.inject({}) { |h, (k, v)|
diff --git a/test/functional/ft_3_participant_registration.rb b/test/functional/ft_3_participant_registration.rb
index 7479eadc..4446ab99 100644
--- a/test/functional/ft_3_participant_registration.rb
+++ b/test/functional/ft_3_participant_registration.rb
@@ -410,5 +410,15 @@ def test_register_block_catchall_default
       %w[ Ruote::StorageParticipant ],
       @engine.participant_list.collect { |pe| pe.classname })
   end
+
+  def test_argument_error_on_instantiated_participant
+
+    assert_raise ArgumentError do
+      @engine.register 'alpha', Ruote::StorageParticipant.new
+    end
+    assert_raise ArgumentError do
+      @engine.register 'alpha', Ruote::StorageParticipant.new, 'hello' => 'kitty'
+    end
+  end
 end