diff --git a/tests/support/server.tcl b/tests/support/server.tcl
index d086366dc..400017c5f 100644
--- a/tests/support/server.tcl
+++ b/tests/support/server.tcl
@@ -159,9 +159,12 @@ proc start_server {options {code undefined}} {
     if {$::external} {
         if {[llength $::servers] == 0} {
             set srv {}
+            # In test_server_main(tests/test_helper.tcl:215~218), increase the value of start_port
+            # and assign it to ::port through the `--port` option, so we need to reduce it.
+            set baseport [expr {$::port-100}]
             dict set srv "host" $::host
-            dict set srv "port" $::port
-            set client [redis $::host $::port 0 $::tls]
+            dict set srv "port" $baseport
+            set client [redis $::host $baseport 0 $::tls]
             dict set srv "client" $client
             $client select 9
 
diff --git a/tests/test_helper.tcl b/tests/test_helper.tcl
index fa5579669..4dbead193 100644
--- a/tests/test_helper.tcl
+++ b/tests/test_helper.tcl
@@ -505,6 +505,9 @@ for {set j 0} {$j < [llength $argv]} {incr j} {
     } elseif {$opt eq {--host}} {
         set ::external 1
         set ::host $arg
+        # If we use an external server, we can only set numclients to 1,
+        # otherwise the port will be miscalculated.
+        set ::numclients 1
         incr j
     } elseif {$opt eq {--port}} {
         set ::port $arg