diff --git a/class.c b/class.c
index 46d44085d5..8e37f8a93e 100644
--- a/class.c
+++ b/class.c
@@ -357,6 +357,15 @@ RMODULE_UNINITIALIZED(VALUE module)
     return RCLASS_SUPER(module) == rb_cBasicObject;
 }
 
+void
+rb_module_set_initialized(VALUE mod)
+{
+    if (RMODULE_UNINITIALIZED(mod)) {
+        RB_OBJ_WRITE(mod, &RCLASS(mod)->super, 0);
+        /* no more re-initialization */
+    }
+}
+
 void
 rb_module_check_initializable(VALUE mod)
 {
@@ -916,10 +925,7 @@ ensure_includable(VALUE klass, VALUE module)
 {
     rb_class_modify_check(klass);
     Check_Type(module, T_MODULE);
-    if (RMODULE_UNINITIALIZED(module)) {
-        RB_OBJ_WRITE(module, &RCLASS(module)->super, 0);
-        /* no more re-initialization */
-    }
+    rb_module_set_initialized(module);
     if (!NIL_P(rb_refinement_module_get_refined_class(module))) {
 	rb_raise(rb_eArgError, "refinement module is not allowed");
     }
diff --git a/eval.c b/eval.c
index 60387f6e06..880c63b88f 100644
--- a/eval.c
+++ b/eval.c
@@ -424,6 +424,9 @@ rb_class_modify_check(VALUE klass)
     if (SPECIAL_CONST_P(klass)) {
 	Check_Type(klass, T_CLASS);
     }
+    if (RB_TYPE_P(klass, T_MODULE)) {
+        rb_module_set_initialized(klass);
+    }
     if (OBJ_FROZEN(klass)) {
 	const char *desc;
 
diff --git a/internal/class.h b/internal/class.h
index 5aaeeb9cdf..a949bfb3e6 100644
--- a/internal/class.h
+++ b/internal/class.h
@@ -115,6 +115,7 @@ int rb_singleton_class_internal_p(VALUE sklass);
 VALUE rb_class_boot(VALUE);
 VALUE rb_class_s_alloc(VALUE klass);
 VALUE rb_module_s_alloc(VALUE klass);
+void rb_module_set_initialized(VALUE module);
 void rb_module_check_initializable(VALUE module);
 VALUE rb_make_metaclass(VALUE, VALUE);
 VALUE rb_include_class_new(VALUE, VALUE);
diff --git a/test/ruby/test_module.rb b/test/ruby/test_module.rb
index 385d8dddfe..429fab897e 100644
--- a/test/ruby/test_module.rb
+++ b/test/ruby/test_module.rb
@@ -446,6 +446,32 @@ class TestModule < Test::Unit::TestCase
     end
   end
 
+  class Bug18185 < Module
+    module InstanceMethods
+    end
+    def initialize
+      include InstanceMethods
+    end
+    class Foo
+      attr_reader :key
+      def initialize(key:)
+        @key = key
+      end
+    end
+  end
+
+  def test_module_subclass_initialize
+    mod = Bug18185.new
+    c = Class.new(Bug18185::Foo) do
+      include mod
+    end
+    anc = c.ancestors
+    assert_include(anc, mod)
+    assert_equal(1, anc.count(BasicObject), ->{anc.inspect})
+    b = c.new(key: 1)
+    assert_equal(1, b.key)
+  end
+
   def test_dup
     OtherSetup.call