diff --git a/lib/shared/messages/sidebar-source-message-spec.js b/lib/shared/messages/sidebar-source-message-spec.js
--- a/lib/shared/messages/sidebar-source-message-spec.js
+++ b/lib/shared/messages/sidebar-source-message-spec.js
@@ -86,6 +86,12 @@
           `${content.sourceMessageID} which is not present in the database`,
       );
     }
+
+    invariant(
+      sourceMessage && !isInvalidSidebarSource(sourceMessage),
+      'SIDEBAR_SOURCE should not point to an invalid message type',
+    );
+
     return {
       type: messageTypes.SIDEBAR_SOURCE,
       id: row.id.toString(),
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
@@ -210,12 +210,32 @@
 // Raw*MessageInfo = used by server, and contained in client's local store
 // *MessageInfo = used by client in UI code
 
+export type ValidRawSidebarSourceMessageInfo =
+  | RawTextMessageInfo
+  | RawCreateThreadMessageInfo
+  | RawAddMembersMessageInfo
+  | RawCreateSubthreadMessageInfo
+  | RawChangeSettingsMessageInfo
+  | RawRemoveMembersMessageInfo
+  | RawChangeRoleMessageInfo
+  | RawLeaveThreadMessageInfo
+  | RawJoinThreadMessageInfo
+  | RawCreateEntryMessageInfo
+  | RawEditEntryMessageInfo
+  | RawDeleteEntryMessageInfo
+  | RawRestoreEntryMessageInfo
+  | RawImagesMessageInfo
+  | RawMediaMessageInfo
+  | RawUpdateRelationshipMessageInfo
+  | RawCreateSidebarMessageInfo
+  | RawUnsupportedMessageInfo;
+
 export type SidebarSourceMessageData = {
   +type: 17,
   +threadID: string,
   +creatorID: string,
   +time: number,
-  +sourceMessage?: RawComposableMessageInfo | RawRobotextMessageInfo,
+  +sourceMessage?: ValidRawSidebarSourceMessageInfo,
 };
 
 export type MessageData =
@@ -369,13 +389,33 @@
   | MultimediaMessageInfo
   | ReactionMessageInfo;
 
+export type ValidSidebarSourceMessageInfo =
+  | TextMessageInfo
+  | CreateThreadMessageInfo
+  | AddMembersMessageInfo
+  | CreateSubthreadMessageInfo
+  | ChangeSettingsMessageInfo
+  | RemoveMembersMessageInfo
+  | ChangeRoleMessageInfo
+  | LeaveThreadMessageInfo
+  | JoinThreadMessageInfo
+  | CreateEntryMessageInfo
+  | EditEntryMessageInfo
+  | DeleteEntryMessageInfo
+  | RestoreEntryMessageInfo
+  | ImagesMessageInfo
+  | MediaMessageInfo
+  | UpdateRelationshipMessageInfo
+  | CreateSidebarMessageInfo
+  | UnsupportedMessageInfo;
+
 export type SidebarSourceMessageInfo = {
   +type: 17,
   +id: string,
   +threadID: string,
   +creator: RelativeUserInfo,
   +time: number,
-  +sourceMessage: ComposableMessageInfo | RobotextMessageInfo,
+  +sourceMessage: ValidSidebarSourceMessageInfo,
 };
 
 export type MessageInfo =