diff --git a/src/emqx_sys_mon.erl b/src/emqx_sys_mon.erl
index 710895bcd..4f07f0b18 100644
--- a/src/emqx_sys_mon.erl
+++ b/src/emqx_sys_mon.erl
@@ -119,7 +119,7 @@ handle_info({monitor, Port, long_schedule, Info}, State) when is_port(Port) ->
     suppress({long_schedule, Port},
              fun() ->
                  WarnMsg = io_lib:format("long_schedule warning: port = ~p, info: ~p", [Port, Info]),
-                 ?LOG(warning, "~s~n~p", [WarnMsg, erlang:port_info(Port)]),
+                 ?LOG(warning, "~s~n~p", [WarnMsg, portinfo(Port)]),
                  safe_publish(long_schedule, WarnMsg)
              end, State);
 
@@ -135,7 +135,7 @@ handle_info({monitor, SusPid, busy_port, Port}, State) ->
     suppress({busy_port, Port},
              fun() ->
                  WarnMsg = io_lib:format("busy_port warning: suspid = ~p, port = ~p", [SusPid, Port]),
-                 ?LOG(warning, "~s~n~p~n~p", [WarnMsg, procinfo(SusPid), erlang:port_info(Port)]),
+                 ?LOG(warning, "~s~n~p~n~p", [WarnMsg, procinfo(SusPid), portinfo(Port)]),
                  safe_publish(busy_port, WarnMsg)
              end, State);
 
@@ -143,7 +143,7 @@ handle_info({monitor, SusPid, busy_dist_port, Port}, State) ->
     suppress({busy_dist_port, Port},
              fun() ->
                  WarnMsg = io_lib:format("busy_dist_port warning: suspid = ~p, port = ~p", [SusPid, Port]),
-                 ?LOG(warning, "~s~n~p~n~p", [WarnMsg, procinfo(SusPid), erlang:port_info(Port)]),
+                 ?LOG(warning, "~s~n~p~n~p", [WarnMsg, procinfo(SusPid), portinfo(Port)]),
                  safe_publish(busy_dist_port, WarnMsg)
              end, State);
 
@@ -200,3 +200,9 @@ safe_publish(Event, WarnMsg) ->
 sysmon_msg(Topic, Payload) ->
     Msg = emqx_message:make(?SYSMON, Topic, Payload),
     emqx_message:set_flag(sys, Msg).
+
+portinfo(Port) ->
+    case is_port(Port) andalso erlang:port_info(Port) of
+        L when is_list(L) -> L;
+        _ -> []
+    end.
diff --git a/test/emqx_sys_mon_SUITE.erl b/test/emqx_sys_mon_SUITE.erl
index 63e7c376a..e177e9527 100644
--- a/test/emqx_sys_mon_SUITE.erl
+++ b/test/emqx_sys_mon_SUITE.erl
@@ -33,6 +33,11 @@
                     {self(), busy_port,
                      concat_str("busy_port warning: suspid = ~p, port = ~p",
                                 self(), list_to_port("#Port<0.4>")), list_to_port("#Port<0.4>")},
+                    %% for the case when the port is missing, for some
+                    %% reason.
+                    {self(), busy_port,
+                     concat_str("busy_port warning: suspid = ~p, port = ~p",
+                                self(), []), []},
                     {self(), busy_dist_port,
                      concat_str("busy_dist_port warning: suspid = ~p, port = ~p",
                                 self(), list_to_port("#Port<0.4>")),list_to_port("#Port<0.4>")},
@@ -122,6 +127,16 @@ t_sys_mon(_Config) ->
               validate_sys_mon_info(PidOrPort, SysMonName,ValidateInfo, InfoOrPort)
       end, ?INPUTINFO).
 
+%% Existing port, but closed.
+t_sys_mon_dead_port(_Config) ->
+    process_flag(trap_exit, true),
+    Port = dead_port(),
+    {PidOrPort, SysMonName, ValidateInfo, InfoOrPort} =
+        {self(), busy_port,
+         concat_str("busy_port warning: suspid = ~p, port = ~p",
+                                self(), Port), Port},
+    validate_sys_mon_info(PidOrPort, SysMonName, ValidateInfo, InfoOrPort).
+
 t_sys_mon2(_Config) ->
     ?SYSMON ! {timeout, ignored, reset},
     ?SYSMON ! {ignored},
@@ -155,3 +170,8 @@ some_function(Parent, _Arg2) ->
         stop ->
             ok
     end.
+
+dead_port() ->
+    Port = erlang:open_port({spawn, "ls"}, []),
+    exit(Port, kill),
+    Port.