diff --git a/lib/types/message-types.js b/lib/types/message-types.js
--- a/lib/types/message-types.js
+++ b/lib/types/message-types.js
@@ -226,7 +226,8 @@
   +creatorID: string,
   +time: number,
   +targetMessageID: string,
-  +reaction: string | null,
+  +reaction: string,
+  +action: 'add_reaction' | 'remove_reaction',
 };
 
 export type MessageData =
@@ -342,7 +343,8 @@
   +creator: RelativeUserInfo,
   +time: number,
   +targetMessageID: string,
-  +reaction: string | null,
+  +reaction: string,
+  +action: 'add_reaction' | 'remove_reaction',
 };
 
 export type MessageInfo =