diff --git a/src/GridFieldAddNewInlineButton.php b/src/GridFieldAddNewInlineButton.php
index 0d888b4..4983d2c 100755
--- a/src/GridFieldAddNewInlineButton.php
+++ b/src/GridFieldAddNewInlineButton.php
@@ -131,7 +131,7 @@ private function getRowTemplate(GridField $grid, GridFieldEditableColumns $edita
                 $field->setName(sprintf(
                     '%s[%s][{%%=o.num%%}][%s]',
                     $grid->getName(),
-                    self::POST_KEY,
+                    GridFieldAddNewInlineButton::POST_KEY,
                     $field->getName()
                 ));
 
@@ -145,7 +145,7 @@ private function getRowTemplate(GridField $grid, GridFieldEditableColumns $edita
                 // Convert GridFieldEditableColumns to the template format
                 $content = str_replace(
                     sprintf('[%s][0]', GridFieldEditableColumns::POST_KEY),
-                    sprintf('[%s][{%%=o.num%%}]', self::POST_KEY),
+                    sprintf('[%s][{%%=o.num%%}]', GridFieldAddNewInlineButton::POST_KEY),
                     $content ?? ''
                 );
             }
@@ -176,7 +176,9 @@ public function handleSave(GridField $grid, DataObjectInterface $record)
         $list  = $grid->getList();
         $value = $grid->Value();
 
-        if (!isset($value[self::POST_KEY]) || !is_array($value[self::POST_KEY])) {
+        if (!isset($value[GridFieldAddNewInlineButton::POST_KEY])
+            || !is_array($value[GridFieldAddNewInlineButton::POST_KEY])
+        ) {
             return;
         }
 
@@ -190,7 +192,7 @@ public function handleSave(GridField $grid, DataObjectInterface $record)
             return;
         }
 
-        foreach ($value[self::POST_KEY] as $fields) {
+        foreach ($value[GridFieldAddNewInlineButton::POST_KEY] as $fields) {
             /** @var DataObject $item */
             $item  = $class::create();
 
diff --git a/src/GridFieldEditableColumns.php b/src/GridFieldEditableColumns.php
index d8cd7fd..b787ea4 100644
--- a/src/GridFieldEditableColumns.php
+++ b/src/GridFieldEditableColumns.php
@@ -121,7 +121,9 @@ public function handleSave(GridField $grid, DataObjectInterface $record)
         $list  = $grid->getList();
         $value = $grid->Value();
 
-        if (!isset($value[self::POST_KEY]) || !is_array($value[self::POST_KEY])) {
+        if (!isset($value[GridFieldEditableColumns::POST_KEY])
+            || !is_array($value[GridFieldEditableColumns::POST_KEY])
+        ) {
             return;
         }
 
@@ -129,13 +131,13 @@ public function handleSave(GridField $grid, DataObjectInterface $record)
         $sortable = $grid->getConfig()->getComponentByType(GridFieldOrderableRows::class);
 
         // Fetch the items before processing them
-        $ids = array_keys($value[self::POST_KEY]);
+        $ids = array_keys($value[GridFieldEditableColumns::POST_KEY]);
         if (empty($ids)) {
             return;
         }
         $itemsCollection = ArrayList::create($list->filter('ID', $ids)->toArray());
 
-        foreach ($value[self::POST_KEY] as $id => $fields) {
+        foreach ($value[GridFieldEditableColumns::POST_KEY] as $id => $fields) {
             if (!is_numeric($id) || !is_array($fields)) {
                 continue;
             }
@@ -325,7 +327,7 @@ protected function getFieldName($name, GridField $grid, DataObjectInterface $rec
         return sprintf(
             '%s[%s][%s][%s]',
             $grid->getName(),
-            self::POST_KEY,
+            GridFieldEditableColumns::POST_KEY,
             $record->ID,
             $name
         );