diff --git a/proxy/vmess/encoding/commands.go b/proxy/vmess/encoding/commands.go
index f1f228f06..ff3ef2f03 100644
--- a/proxy/vmess/encoding/commands.go
+++ b/proxy/vmess/encoding/commands.go
@@ -139,7 +139,7 @@ func (f *CommandSwitchAccountFactory) Unmarshal(data []byte) (interface{}, error
 	}
 	cmd.Level = uint32(data[levelStart])
 	timeStart := levelStart + 1
-	if len(data) < timeStart {
+	if len(data) < timeStart+1 {
 		return nil, newError("insufficient length.")
 	}
 	cmd.ValidMin = data[timeStart]
diff --git a/proxy/vmess/encoding/commands_test.go b/proxy/vmess/encoding/commands_test.go
index f22a24777..4c85bfcb5 100644
--- a/proxy/vmess/encoding/commands_test.go
+++ b/proxy/vmess/encoding/commands_test.go
@@ -1,6 +1,7 @@
 package encoding_test
 
 import (
+	"github.com/stretchr/testify/assert"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -35,3 +36,23 @@ func TestSwitchAccount(t *testing.T) {
 		t.Error(r)
 	}
 }
+
+func TestSwitchAccountBugOffByOne(t *testing.T) {
+	sa := &protocol.CommandSwitchAccount{
+		Port:     1234,
+		ID:       uuid.New(),
+		AlterIds: 1024,
+		Level:    128,
+		ValidMin: 16,
+	}
+
+	buffer := buf.New()
+	csaf := CommandSwitchAccountFactory{}
+	common.Must(csaf.Marshal(sa, buffer))
+
+	Payload := buffer.Bytes()
+
+	cmd, err := csaf.Unmarshal(Payload[:len(Payload)-1])
+	assert.Error(t, err)
+	assert.Nil(t, cmd)
+}