diff --git a/src/emqtt.erl b/src/emqtt.erl
index 181427a..aaebb81 100644
--- a/src/emqtt.erl
+++ b/src/emqtt.erl
@@ -88,7 +88,7 @@
                 | {force_ping, boolean()}
                 | {properties, properties()}).
 
--type(maybe(T) :: undefined | T).
+-type(optional(T) :: undefined | T).
 -type(topic() :: binary()).
 -type(payload() :: iodata()).
 -type(packet_id() :: 0..16#FFFF).
@@ -128,12 +128,12 @@
           bridge_mode     :: boolean(),
           clientid        :: binary(),
           clean_start     :: boolean(),
-          username        :: maybe(binary()),
-          password        :: maybe(binary()),
+          username        :: optional(binary()),
+          password        :: optional(binary()),
           proto_ver       :: version(),
           proto_name      :: iodata(),
           keepalive       :: non_neg_integer(),
-          keepalive_timer :: maybe(reference()),
+          keepalive_timer :: optional(reference()),
           force_ping      :: boolean(),
           paused          :: boolean(),
           will_flag       :: boolean(),