diff --git a/lib/shared/dm-ops/add-viewer-to-thread-members-spec.js b/lib/shared/dm-ops/add-viewer-to-thread-members-spec.js
--- a/lib/shared/dm-ops/add-viewer-to-thread-members-spec.js
+++ b/lib/shared/dm-ops/add-viewer-to-thread-members-spec.js
@@ -48,9 +48,9 @@
       const messageData =
         createAddViewerToThreadMembersMessageDataFromDMOp(dmOperation);
 
-      const rawMessageInfos = [
-        rawMessageInfoFromMessageData(messageData, messageID),
-      ];
+      const rawMessageInfos = messageID
+        ? [rawMessageInfoFromMessageData(messageData, messageID)]
+        : [];
 
       const threadID = existingThreadDetails.threadID;
       const currentThreadInfo = utilities.threadInfos[threadID];
diff --git a/lib/types/dm-ops.js b/lib/types/dm-ops.js
--- a/lib/types/dm-ops.js
+++ b/lib/types/dm-ops.js
@@ -200,36 +200,38 @@
 type DMAddMembersBase = {
   +editorID: string,
   +time: number,
-  +messageID: string,
   +addedUserIDs: $ReadOnlyArray<string>,
 };
 const dmAddMembersBaseValidatorShape = {
   editorID: tUserID,
   time: t.Number,
-  messageID: t.String,
   addedUserIDs: t.list(tUserID),
 };
 
 export type DMAddMembersOperation = $ReadOnly<{
   +type: 'add_members',
   +threadID: string,
+  +messageID: string,
   ...DMAddMembersBase,
 }>;
 export const dmAddMembersOperationValidator: TInterface<DMAddMembersOperation> =
   tShape<DMAddMembersOperation>({
     type: tString(dmOperationTypes.ADD_MEMBERS),
     threadID: t.String,
+    messageID: t.String,
     ...dmAddMembersBaseValidatorShape,
   });
 
 export type DMAddViewerToThreadMembersOperation = $ReadOnly<{
   +type: 'add_viewer_to_thread_members',
+  +messageID: ?string,
   +existingThreadDetails: CreateThickRawThreadInfoInput,
   ...DMAddMembersBase,
 }>;
 export const dmAddViewerToThreadMembersValidator: TInterface<DMAddViewerToThreadMembersOperation> =
   tShape<DMAddViewerToThreadMembersOperation>({
     type: tString(dmOperationTypes.ADD_VIEWER_TO_THREAD_MEMBERS),
+    messageID: t.maybe(t.String),
     existingThreadDetails: createThickRawThreadInfoInputValidator,
     ...dmAddMembersBaseValidatorShape,
   });