diff --git a/proc.c b/proc.c
index 667b86ac1b..ca6e671fd6 100644
--- a/proc.c
+++ b/proc.c
@@ -2068,7 +2068,7 @@ rb_mod_define_method(int argc, VALUE *argv, VALUE mod)
 	}
 	else {
 	    rb_raise(rb_eTypeError,
-		     "wrong argument type %s (expected Proc/Method)",
+		     "wrong argument type %s (expected Proc/Method/UnboundMethod)",
 		     rb_obj_classname(body));
 	}
     }