diff --git a/proc.c b/proc.c
index eafcf336bf..813390840e 100644
--- a/proc.c
+++ b/proc.c
@@ -1288,7 +1288,7 @@ rb_mod_define_method(int argc, VALUE *argv, VALUE mod)
  *     define_singleton_method(symbol) { block } => proc
  *
  *  Defines a singleton method in the receiver. The _method_
- *  parameter can be a +Proc+ or +Method+ object.
+ *  parameter can be a +Proc+, a +Method+ or an +UnboundMethod+ object.
  *  If a block is specified, it is used as the method body. 
  *
  *     class A