diff --git a/app/Models/Invoice.php b/app/Models/Invoice.php
index aab0f39..9f113d6 100644
--- a/app/Models/Invoice.php
+++ b/app/Models/Invoice.php
@@ -251,7 +251,7 @@ class Invoice extends Model
         static::creating(function (self $model) {
             $model->ref        = \sprintf(
                 '%s_%s_%s',
-                Carbon::now()->isoFormat('YYYY'),
+                Carbon::now()->isoFormat('YYYY-MM'),
                 Str::padLeft((Invoice::whereYear('created_at', Carbon::now()->isoFormat('YYYY'))
                     ->whereMonth('created_at', Carbon::now()->isoFormat('MM'))
                 ->count() + 1) ?: 1, 4, '0'),