diff --git a/ext/date/date_core.c b/ext/date/date_core.c
index 4fd35c978a..ab00f67778 100644
--- a/ext/date/date_core.c
+++ b/ext/date/date_core.c
@@ -4768,8 +4768,28 @@ d_lite_initialize_copy(VALUE copy, VALUE date)
     {
 	get_d2(copy, date);
 	if (simple_dat_p(bdat)) {
-	    adat->s = bdat->s;
-	    adat->s.flags &= ~COMPLEX_DAT;
+	    if (simple_dat_p(adat)) {
+		adat->s = bdat->s;
+	    }
+	    else {
+		adat->c.flags = bdat->s.flags | COMPLEX_DAT;
+		adat->c.nth = bdat->s.nth;
+		adat->c.jd = bdat->s.jd;
+		adat->c.df = 0;
+		adat->c.sf = INT2FIX(0);
+		adat->c.of = 0;
+		adat->c.sg = bdat->s.sg;
+		adat->c.year = bdat->s.year;
+#ifndef USE_PACK
+		adat->c.mon = bdat->s.mon;
+		adat->c.mday = bdat->s.mday;
+		adat->c.hour = bdat->s.hour;
+		adat->c.min = bdat->s.min;
+		adat->c.sec = bdat->s.sec;
+#else
+		adat->c.pc = bdat->s.pc;
+#endif
+	    }
 	}
 	else {
 	    if (!complex_dat_p(adat))
@@ -4777,7 +4797,6 @@ d_lite_initialize_copy(VALUE copy, VALUE date)
 			 "cannot load complex into simple");
 
 	    adat->c = bdat->c;
-	    adat->c.flags |= COMPLEX_DAT;
 	}
     }
     return copy;
diff --git a/test/date/test_date_new.rb b/test/date/test_date_new.rb
index fe9a631ad0..6ea9c69041 100644
--- a/test/date/test_date_new.rb
+++ b/test/date/test_date_new.rb
@@ -267,4 +267,12 @@ class TestDateNew < Test::Unit::TestCase
     assert_in_delta(t, t2, t - z + 2)
   end
 
+  def test_memsize
+    require 'objspace'
+    t = DateTime.now
+    size = ObjectSpace.memsize_of(t)
+    t.__send__(:initialize_copy, Date.today)
+    assert_instance_of(DateTime, t)
+    assert_equal(size, ObjectSpace.memsize_of(t), "not reallocated but memsize changed")
+  end
 end