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
@@ -9,6 +9,7 @@
   generatePendingThreadColor,
   generateRandomColor,
 } from 'lib/shared/color-utils.js';
+import { isInvalidSidebarSource } from 'lib/shared/message-utils.js';
 import { getThreadTypeParentRequirement } from 'lib/shared/thread-utils.js';
 import type { Shape } from 'lib/types/core.js';
 import { messageTypes } from 'lib/types/message-types-enum.js';
@@ -194,14 +195,7 @@
     validateMembers: { initialMemberIDs, ghostMemberIDs },
   } = await promiseAll(checkPromises);
 
-  if (
-    sourceMessage &&
-    (sourceMessage.type === messageTypes.REACTION ||
-      sourceMessage.type === messageTypes.EDIT_MESSAGE ||
-      sourceMessage.type === messageTypes.SIDEBAR_SOURCE ||
-      sourceMessage.type === messageTypes.TOGGLE_PIN ||
-      sourceMessage.type === messageTypes.CHANGE_ROLE)
-  ) {
+  if (sourceMessage && isInvalidSidebarSource(sourceMessage.type)) {
     throw new ServerError('invalid_parameters');
   }
 
@@ -425,6 +419,14 @@
     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 &&
+        sourceMessage.type !== messageTypes.CHANGE_ROLE,
+      'Invalid sidebar source type',
+    );
 
     let editedSourceMessage = sourceMessage;
     if (sourceMessageID && sourceMessage.type === messageTypes.TEXT) {
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
@@ -642,6 +642,16 @@
   return `${localIDPrefix}${nextLocalID}`;
 }
 
+function isInvalidSidebarSource(messageType: number): boolean {
+  return (
+    messageType === messageTypes.REACTION ||
+    messageType === messageTypes.EDIT_MESSAGE ||
+    messageType === messageTypes.SIDEBAR_SOURCE ||
+    messageType === messageTypes.TOGGLE_PIN ||
+    messageType === messageTypes.CHANGE_ROLE
+  );
+}
+
 export {
   localIDPrefix,
   messageKey,
@@ -671,4 +681,5 @@
   modifyItemForResultScreen,
   constructChangeRoleEntityText,
   useNextLocalID,
+  isInvalidSidebarSource,
 };
diff --git a/lib/shared/message-utils.test.js b/lib/shared/message-utils.test.js
new file mode 100644
--- /dev/null
+++ b/lib/shared/message-utils.test.js
@@ -0,0 +1,24 @@
+// @flow
+
+import { isInvalidSidebarSource } from './message-utils.js';
+import { messageTypes } from '../types/message-types-enum.js';
+import { values } from '../utils/objects.js';
+
+describe('isInvalidSidebarSource', () => {
+  it('should return true for all message types except for the ones listed', () => {
+    values(messageTypes).forEach(messageType => {
+      const shouldBeInvalidSidebarSource = isInvalidSidebarSource(messageType);
+      if (
+        messageType === messageTypes.REACTION ||
+        messageType === messageTypes.EDIT_MESSAGE ||
+        messageType === messageTypes.SIDEBAR_SOURCE ||
+        messageType === messageTypes.TOGGLE_PIN ||
+        messageType === messageTypes.CHANGE_ROLE
+      ) {
+        expect(shouldBeInvalidSidebarSource).toBe(true);
+      } else {
+        expect(shouldBeInvalidSidebarSource).toBe(false);
+      }
+    });
+  });
+});
diff --git a/lib/shared/thread-utils.js b/lib/shared/thread-utils.js
--- a/lib/shared/thread-utils.js
+++ b/lib/shared/thread-utils.js
@@ -9,7 +9,7 @@
 import { generatePendingThreadColor } from './color-utils.js';
 import { type ParserRules } from './markdown.js';
 import { extractMentionsFromText } from './mention-utils.js';
-import { getMessageTitle } from './message-utils.js';
+import { getMessageTitle, isInvalidSidebarSource } from './message-utils.js';
 import { relationshipBlockedInEitherDirection } from './relationship-utils.js';
 import threadWatcher from './thread-watcher.js';
 import {
@@ -1254,17 +1254,10 @@
     state => state.userStore.userInfos[messageInfo.creator.id],
   );
 
-  if (!messageInfo.id || threadInfo.sourceMessageID === messageInfo.id) {
-    return false;
-  }
-
-  const { type } = messageInfo;
   if (
-    type === messageTypes.REACTION ||
-    type === messageTypes.EDIT_MESSAGE ||
-    type === messageTypes.SIDEBAR_SOURCE ||
-    type === messageTypes.TOGGLE_PIN ||
-    type === messageTypes.CHANGE_ROLE
+    !messageInfo.id ||
+    threadInfo.sourceMessageID === messageInfo.id ||
+    isInvalidSidebarSource(messageInfo.type)
   ) {
     return false;
   }