diff --git a/library/think/Model.php b/library/think/Model.php
index ae4f2993..47fdc502 100644
--- a/library/think/Model.php
+++ b/library/think/Model.php
@@ -367,6 +367,10 @@ abstract class Model implements \JsonSerializable, \ArrayAccess
      */
     protected function writeTransform($value, $type)
     {
+        if (is_null($value)) {
+            return null;
+        }
+
         if (is_array($type)) {
             list($type, $param) = $type;
         } elseif (strpos($type, ':')) {
@@ -475,6 +479,10 @@ abstract class Model implements \JsonSerializable, \ArrayAccess
      */
     protected function readTransform($value, $type)
     {
+        if (is_null($value)) {
+            return null;
+        }
+
         if (is_array($type)) {
             list($type, $param) = $type;
         } elseif (strpos($type, ':')) {