diff --git a/src/fd_ws.erl b/src/fd_ws.erl
index 365e3da..81c2898 100644
--- a/src/fd_ws.erl
+++ b/src/fd_ws.erl
@@ -23,7 +23,7 @@
                    | {timeout, RestData :: binary() | erlang:iovec()}
                    | inet:posix().
 
--define(MAX_PAYLOAD_SIZE, (1 bsl 63)).
+-define(MAX_PAYLOAD_SIZE, ((1 bsl 63) - 1)).
 
 %% Frames
 %% https://datatracker.ietf.org/doc/html/rfc6455#section-5.2
@@ -716,7 +716,7 @@ payload_to_binary(X)                       -> unicode:characters_to_binary(X).
 message_to_frame(Data, Payload)
         when ((Data =:= text) orelse (Data =:= binary)),
              is_binary(Payload),
-             (byte_size(Payload) < ?MAX_PAYLOAD_SIZE) ->
+             (byte_size(Payload) =< ?MAX_PAYLOAD_SIZE) ->
     #frame{fin            = true,
            opcode         = Data,
            payload_length = byte_size(Payload),
@@ -817,5 +817,5 @@ render_payload_length(Len) when 0 =< Len, Len =< 125 ->
     <<Len:7>>;
 render_payload_length(Len) when 126 =< Len, Len =< 2#1111_1111_1111_1111 ->
     <<126:7, Len:16>>;
-render_payload_length(Len) when (1 bsl 16) =< Len, Len < ?MAX_PAYLOAD_SIZE ->
+render_payload_length(Len) when (1 bsl 16) =< Len, Len =< ?MAX_PAYLOAD_SIZE ->
     <<127:7, Len:64>>.