diff --git a/ChangeLog b/ChangeLog
index d4ff94d61f..6143191645 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Fri Sep 27 01:05:00 2013  Zachary Scott  <e@zzak.io>
+
+	* re.c: [DOC] arguments of Regexp::union receive #to_regexp [Bug #8205]
+
 Fri Sep 27 00:39:27 2013  Zachary Scott  <e@zzak.io>
 
 	* struct.c: [DOC] grammar of ArgumentError in Struct.new [Bug #8936]
diff --git a/re.c b/re.c
index 9954805a9a..a7074d8282 100644
--- a/re.c
+++ b/re.c
@@ -3276,6 +3276,9 @@ rb_reg_s_union(VALUE self, VALUE args0)
  *     Regexp.union("skiing", "sledding")   #=> /skiing|sledding/
  *     Regexp.union(["skiing", "sledding"]) #=> /skiing|sledding/
  *     Regexp.union(/dogs/, /cats/i)        #=> /(?-mix:dogs)|(?i-mx:cats)/
+ *
+ *  Note: the arguments for ::union will try to be converted into a regular
+ *  expression literal via #to_regexp.
  */
 static VALUE
 rb_reg_s_union_m(VALUE self, VALUE args)