diff --git a/keyserver/src/creators/thread-creator.js b/keyserver/src/creators/thread-creator.js
--- a/keyserver/src/creators/thread-creator.js
+++ b/keyserver/src/creators/thread-creator.js
@@ -8,7 +8,7 @@
   generatePendingThreadColor,
   generateRandomColor,
 } from 'lib/shared/color-utils.js';
-import { isInvalidSidebarSource } from 'lib/shared/message-utils.js';
+import { isValidRawSidebarSourceMessageInfo } from 'lib/shared/message-utils.js';
 import { getThreadTypeParentRequirement } from 'lib/shared/thread-utils.js';
 import { threadTypeIsCommunityRoot } from 'lib/shared/threads/thread-specs.js';
 import { messageTypes } from 'lib/types/message-types-enum.js';
@@ -203,7 +203,7 @@
     confirmParentPermission: confirmParentPermissionPromise,
   });
 
-  if (sourceMessage && isInvalidSidebarSource(sourceMessage)) {
+  if (sourceMessage && !isValidRawSidebarSourceMessageInfo(sourceMessage)) {
     throw new ServerError('invalid_parameters');
   }
 
@@ -438,13 +438,6 @@
     if (!sourceMessage) {
       throw new ServerError('invalid_parameters');
     }
-    invariant(
-      sourceMessage.type !== messageTypes.REACTION &&
-        sourceMessage.type !== messageTypes.EDIT_MESSAGE &&
-        sourceMessage.type !== messageTypes.SIDEBAR_SOURCE &&
-        sourceMessage.type !== messageTypes.TOGGLE_PIN,
-      'Invalid sidebar source type',
-    );
 
     let editedSourceMessage = sourceMessage;
     if (sourceMessageID && sourceMessage.type === messageTypes.TEXT) {
diff --git a/lib/shared/dm-ops/create-sidebar-spec.js b/lib/shared/dm-ops/create-sidebar-spec.js
--- a/lib/shared/dm-ops/create-sidebar-spec.js
+++ b/lib/shared/dm-ops/create-sidebar-spec.js
@@ -21,7 +21,7 @@
 import { updateTypes } from '../../types/update-types-enum.js';
 import { generatePendingThreadColor } from '../color-utils.js';
 import {
-  isInvalidSidebarSource,
+  isValidRawSidebarSourceMessageInfo,
   rawMessageInfoFromMessageData,
 } from '../message-utils.js';
 import { createThreadTimestamps } from '../thread-utils.js';
@@ -52,7 +52,7 @@
         'joined thick thread ${parentThreadID} after its creation',
     );
   }
-  if (isInvalidSidebarSource(sourceMessage)) {
+  if (!isValidRawSidebarSourceMessageInfo(sourceMessage)) {
     throw new Error(
       `sourceMessage ${sourceMessageID} is an invalid sidebar source`,
     );
diff --git a/lib/shared/message-utils.js b/lib/shared/message-utils.js
--- a/lib/shared/message-utils.js
+++ b/lib/shared/message-utils.js
@@ -39,6 +39,8 @@
   type RawMultimediaMessageInfo,
   type RobotextMessageInfo,
   type ThreadMessageInfo,
+  type ValidRawSidebarSourceMessageInfo,
+  type ValidSidebarSourceMessageInfo,
 } from '../types/message-types.js';
 import type { DeleteMessageInfo } from '../types/messages/delete.js';
 import type { EditMessageInfo } from '../types/messages/edit.js';
@@ -518,6 +520,32 @@
   );
 }
 
+function isValidRawSidebarSourceMessageInfo(
+  message: RawMessageInfo,
+): implies message is ValidRawSidebarSourceMessageInfo {
+  return (
+    message.type !== messageTypes.REACTION &&
+    message.type !== messageTypes.EDIT_MESSAGE &&
+    message.type !== messageTypes.SIDEBAR_SOURCE &&
+    message.type !== messageTypes.TOGGLE_PIN &&
+    message.type !== messageTypes.DELETE_MESSAGE &&
+    messageSpecs[message.type].canBeSidebarSource
+  );
+}
+
+function isValidSidebarSourceMessageInfo(
+  message: MessageInfo,
+): implies message is ValidSidebarSourceMessageInfo {
+  return (
+    message.type !== messageTypes.REACTION &&
+    message.type !== messageTypes.EDIT_MESSAGE &&
+    message.type !== messageTypes.SIDEBAR_SOURCE &&
+    message.type !== messageTypes.TOGGLE_PIN &&
+    message.type !== messageTypes.DELETE_MESSAGE &&
+    messageSpecs[message.type].canBeSidebarSource
+  );
+}
+
 // Prefer checking isInvalidPinSourceForThread below. This function doesn't
 // check whether the user is attempting to pin a SIDEBAR_SOURCE in the context
 // of its parent thread, so it's not suitable for permission checks. We only
@@ -665,6 +693,8 @@
   modifyItemForResultScreen,
   constructChangeRoleEntityText,
   isInvalidSidebarSource,
+  isValidRawSidebarSourceMessageInfo,
+  isValidSidebarSourceMessageInfo,
   isInvalidPinSource,
   isInvalidPinSourceForThread,
   isUnableToBeRenderedIndependently,
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
@@ -19,7 +19,10 @@
 } from '../../types/message-types.js';
 import type { NotifTexts } from '../../types/notif-types.js';
 import type { RelativeUserInfo } from '../../types/user-types.js';
-import { isInvalidSidebarSource } from '../message-utils.js';
+import {
+  isValidSidebarSourceMessageInfo,
+  isValidRawSidebarSourceMessageInfo,
+} from '../message-utils.js';
 
 export const sidebarSourceMessageSpec: MessageSpec<
   SidebarSourceMessageData,
@@ -77,7 +80,7 @@
 
     const content = JSON.parse(row.content);
     let sourceMessage = derivedMessages.get(content.sourceMessageID);
-    if (!sourceMessage || isInvalidSidebarSource(sourceMessage)) {
+    if (!sourceMessage || !isValidRawSidebarSourceMessageInfo(sourceMessage)) {
       console.warn(
         `Message with id ${row.id} has a derived message ` +
           `${content.sourceMessageID} which is not present in the database`,
@@ -108,7 +111,7 @@
       rawMessageInfo.sourceMessage,
     );
     invariant(
-      sourceMessage && !isInvalidSidebarSource(sourceMessage),
+      sourceMessage && isValidSidebarSourceMessageInfo(sourceMessage),
       'Sidebars can not be created from SIDEBAR SOURCE, REACTION, EDIT OR PIN MESSAGE',
     );