diff --git a/app/Http/Requests/CreatePaymentRequest.php b/app/Http/Requests/CreatePaymentRequest.php
index f836f0a5a384..52e9d313bf9f 100644
--- a/app/Http/Requests/CreatePaymentRequest.php
+++ b/app/Http/Requests/CreatePaymentRequest.php
@@ -29,7 +29,7 @@ class CreatePaymentRequest extends Request
         $rules = array(
             'client' => 'required',
             'invoice' => 'required',
-            'amount' => 'required|less_than:{$invoice->balance}|positive',
+            'amount' => "required|less_than:{$invoice->balance}|positive",
         );
 
         if ($input['payment_type_id'] == PAYMENT_TYPE_CREDIT) {