Raise an exception if an initializer is defined without a block

railties/lib/rails/initializable.rb

@@ -93,6 +93,7 @@ module Rails
       end
 
       def initializer(name, opts = {}, &blk)
+        raise ArgumentError, "A block must be passed when defining an initializer" unless blk
         @initializers ||= []
         @initializers << Initializer.new(name, nil, opts, &blk)
       end

railties/test/initializable_test.rb

@@ -150,6 +150,16 @@ module InitializableTests
       Word.run_initializers
       assert_equal "bird", $word
     end
+
+    test "creating initializer without a block raises an error" do
+      assert_raise(ArgumentError) do
+        Class.new do
+          include Rails::Initializable
+
+          initializer :foo
+        end
+      end
+    end
   end
 
   class BeforeAfter < ActiveSupport::TestCase