diff --git a/apps/emqx/src/emqx_sys_mon.erl b/apps/emqx/src/emqx_sys_mon.erl
index 1f8f04d97..1df9ec604 100644
--- a/apps/emqx/src/emqx_sys_mon.erl
+++ b/apps/emqx/src/emqx_sys_mon.erl
@@ -202,7 +202,12 @@ get_proc_lib_initial_call(Pid) ->
     end.
 
 portinfo(Port) ->
-    [{port, Port} | erlang:port_info(Port)].
+    PortInfo =
+        case is_port(Port) andalso erlang:port_info(Port) of
+            L when is_list(L) -> L;
+            _ -> []
+        end,
+    [{port, Port} | PortInfo].
 
 safe_publish(Event, WarnMsg) ->
     Topic = emqx_topic:systop(lists:concat(['sysmon/', Event])),
diff --git a/apps/emqx/test/emqx_sys_mon_SUITE.erl b/apps/emqx/test/emqx_sys_mon_SUITE.erl
index 8e4bd65db..338188c21 100644
--- a/apps/emqx/test/emqx_sys_mon_SUITE.erl
+++ b/apps/emqx/test/emqx_sys_mon_SUITE.erl
@@ -35,6 +35,11 @@
                     {self(), busy_port,
                      fmt("busy_port warning: suspid = ~p, port = ~p",
                          [self(), ?FAKE_PORT]), ?FAKE_PORT},
+                    %% for the case when the port is missing, for some
+                    %% reason.
+                    {self(), busy_port,
+                     fmt("busy_port warning: suspid = ~p, port = ~p",
+                         [self(), []]), []},
                     {self(), busy_dist_port,
                      fmt("busy_dist_port warning: suspid = ~p, port = ~p",
                          [self(), ?FAKE_PORT]), ?FAKE_PORT},
@@ -120,6 +125,16 @@ 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,
+         fmt("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},
@@ -151,3 +166,8 @@ some_function(Parent, _Arg2) ->
         stop ->
             ok
     end.
+
+dead_port() ->
+    Port = erlang:open_port({spawn, "ls"}, []),
+    exit(Port, kill),
+    Port.