diff --git a/td/generate/TlDocumentationGenerator.php b/td/generate/TlDocumentationGenerator.php
index b1bde1d4..61101e1c 100644
--- a/td/generate/TlDocumentationGenerator.php
+++ b/td/generate/TlDocumentationGenerator.php
@@ -242,7 +242,7 @@ abstract class TlDocumentationGenerator
                 $this->addClassDocumentation($class_name, $base_class_name, $class_description);
 
                 foreach ($known_fields as $name => $field_type) {
-                    $may_be_null = stripos($info[$field_name], 'may be null') !== false;
+                    $may_be_null = stripos($info[$name], 'may be null') !== false;
                     $field_name = $this->getFieldName($name, $class_name);
                     $field_type_name = $this->getTypeName($field_type);
                     $this->addFieldDocumentation($class_name, $field_name, $field_type_name, $info[$name], $may_be_null);