diff --git a/smug.go b/smug.go
index 2026926..4b683b9 100644
--- a/smug.go
+++ b/smug.go
@@ -132,7 +132,12 @@ func (smug Smug) Start(config Config, windows []string) error {
 	}
 
 	if len(windows) == 0 {
-		err = smug.tmux.KillWindow(ses + "0")
+		windows, err := smug.tmux.ListWindows(ses)
+		if err != nil {
+			return err
+		}
+
+		err = smug.tmux.KillWindow(ses + windows[0])
 		if err != nil {
 			return err
 		}
diff --git a/smug_test.go b/smug_test.go
index d8c65cb..696869f 100644
--- a/smug_test.go
+++ b/smug_test.go
@@ -24,7 +24,8 @@ var testTable = []struct {
 			"/bin/sh -c command1",
 			"/bin/sh -c command2",
 			"tmux new -Pd -s ses",
-			"tmux kill-window -t ses:0",
+			"tmux list-windows -t ses: -F #{window_index}",
+			"tmux kill-window -t ses:ses:",
 			"tmux move-window -r",
 			"tmux attach -t ses:0",
 		},
@@ -65,7 +66,8 @@ var testTable = []struct {
 			"tmux neww -Pd -t ses: -n win1 -c root",
 			"tmux split-window -Pd -t ses: -c root -h",
 			"tmux select-layout -t ses:win1 main-horizontal",
-			"tmux kill-window -t ses:0",
+			"tmux list-windows -t ses: -F #{window_index}",
+			"tmux kill-window -t ses:ses:",
 			"tmux move-window -r",
 			"tmux attach -t ses:0",
 		},
diff --git a/tmux.go b/tmux.go
index b4e997b..f283c66 100644
--- a/tmux.go
+++ b/tmux.go
@@ -3,6 +3,7 @@ package main
 import (
 	"os"
 	"os/exec"
+	"strings"
 )
 
 const (
@@ -115,3 +116,14 @@ func (tmux Tmux) StopSession(target string) (string, error) {
 	cmd := exec.Command("tmux", "kill-session", "-t", target)
 	return tmux.commander.Exec(cmd)
 }
+
+func (tmux Tmux) ListWindows(target string) ([]string, error) {
+	cmd := exec.Command("tmux", "list-windows", "-t", target, "-F", "#{window_index}")
+
+	output, err := tmux.commander.Exec(cmd)
+	if err != nil {
+		return []string{}, err
+	}
+
+	return strings.Split(output, "\n"), nil
+}