diff --git a/src/field_set.ts b/src/field_set.ts
index 8a5e79c..810456e 100644
--- a/src/field_set.ts
+++ b/src/field_set.ts
@@ -4,6 +4,7 @@ import {Attachment} from './attachment';
 export interface FieldSet {
     [key: string]:
         | undefined
+        | null
         | string
         | number
         | boolean