diff --git a/net-mqtt.cabal b/net-mqtt.cabal
--- a/net-mqtt.cabal
+++ b/net-mqtt.cabal
@@ -1,11 +1,11 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.38.1.
+-- This file has been generated from package.yaml by hpack version 0.38.3.
 --
 -- see: https://github.com/sol/hpack
 
 name:           net-mqtt
-version:        0.8.6.4
+version:        0.8.6.5
 synopsis:       An MQTT Protocol Implementation.
 description:    Please see the README on GitHub at <https://github.com/dustin/mqtt-hs#readme>
 category:       Network
diff --git a/src/Network/MQTT/Types.hs b/src/Network/MQTT/Types.hs
--- a/src/Network/MQTT/Types.hs
+++ b/src/Network/MQTT/Types.hs
@@ -847,9 +847,11 @@
 parseAuth :: A.Parser MQTTPkt
 parseAuth = do
   _ <- A.word8 0xf0
-  _ <- parseHdrLen
-  r <- AuthRequest <$> A.anyWord8 <*> parseProperties Protocol50
-  pure $ AuthPkt r
+  rl <- parseHdrLen
+  -- With a Remaining Length of 0, the reason code is 0x00 (Success).
+  r <- if rl > 0 then A.anyWord8 else pure 0
+  props <- if rl > 1 then parseProperties Protocol50 else pure mempty
+  pure $ AuthPkt (AuthRequest r props)
 
 data DiscoReason = DiscoNormalDisconnection
   | DiscoDisconnectWithWill
@@ -935,7 +937,8 @@
 parseDisconnect Protocol50 = do
   _ <- A.word8 0xe0
   rl <- parseHdrLen
-  r <- A.anyWord8
+  -- With a Remaining Length of 0, the reason code is 0x00 (Normal disconnection).
+  r <- if rl > 0 then A.anyWord8 else pure 0
   props <- if rl > 1 then parseProperties Protocol50 else pure mempty
 
   pure $ DisconnectPkt (DisconnectRequest (fromByte r) props)
diff --git a/test/Spec.hs b/test/Spec.hs
--- a/test/Spec.hs
+++ b/test/Spec.hs
@@ -151,3 +151,27 @@
   testGroup "topic matching" testTopicMatching,
   testProperty "arbitrary topic matching" prop_TopicMatching
   ]
+
+-- | Parse exactly the given bytes as a single MQTT 5 packet.
+parses50 :: [Word8] -> MQTTPkt -> Assertion
+parses50 bs want = case A.parse (parsePacket Protocol50) (L.pack bs) of
+  A.Done "" got -> assertEqual (show bs) want got
+  x             -> assertFailure (show bs <> ": " <> show x)
+
+-- A DISCONNECT with a Remaining Length of 0 has an implied reason code
+-- of 0x00 (Normal disconnection) and no properties [MQTT 5 §3.14.2.1].
+unit_disconnectShort50 :: Assertion
+unit_disconnectShort50 = parses50 [0xe0, 0x00] (DisconnectPkt (DisconnectRequest DiscoNormalDisconnection []))
+
+-- A Remaining Length of 1 is a reason code with no properties [MQTT 5 §3.14.2.2.1].
+unit_disconnectReasonOnly50 :: Assertion
+unit_disconnectReasonOnly50 = parses50 [0xe0, 0x01, 0x04] (DisconnectPkt (DisconnectRequest DiscoDisconnectWithWill []))
+
+-- An AUTH with a Remaining Length of 0 has an implied reason code of
+-- 0x00 (Success) and no properties [MQTT 5 §3.15.2.1].
+unit_authShort50 :: Assertion
+unit_authShort50 = parses50 [0xf0, 0x00] (AuthPkt (AuthRequest 0x00 []))
+
+-- A Remaining Length of 1 is a reason code with no properties [MQTT 5 §3.15.2.2.1].
+unit_authReasonOnly50 :: Assertion
+unit_authReasonOnly50 = parses50 [0xf0, 0x01, 0x18] (AuthPkt (AuthRequest 0x18 []))