diff --git a/src/Providers/EncodeServiceProvider.php b/src/Providers/EncodeServiceProvider.php
index 19e0da4..7e6aa41 100644
--- a/src/Providers/EncodeServiceProvider.php
+++ b/src/Providers/EncodeServiceProvider.php
@@ -15,8 +15,8 @@ class EncodeServiceProvider implements ServiceProviderContract
 {
     public function register(Kernel $kernel)
     {
-        $kernel->bind('encoding',function (string $outputEncoding,string $inputEncoding = null){
+        $kernel->bind('encoding',function (string $outputEncoding,?string $inputEncoding = null){
             return EncodeService::convert($this,$outputEncoding,$inputEncoding);
         });
     }
-}
\ No newline at end of file
+}