diff --git a/keyserver/src/updaters/thread-permission-updaters.js b/keyserver/src/updaters/thread-permission-updaters.js
--- a/keyserver/src/updaters/thread-permission-updaters.js
+++ b/keyserver/src/updaters/thread-permission-updaters.js
@@ -20,8 +20,8 @@
 } from 'lib/types/thread-permission-types.js';
 import { threadPermissions } from 'lib/types/thread-permission-types.js';
 import {
-  type ThreadType,
-  assertThreadType,
+  type ThinThreadType,
+  assertThinThreadType,
 } from 'lib/types/thread-types-enum.js';
 import { updateTypes } from 'lib/types/update-types-enum.js';
 import {
@@ -343,7 +343,7 @@
 type RoleThreadResult = {
   +roleColumnValue: string,
   +depth: number,
-  +threadType: ThreadType,
+  +threadType: ThinThreadType,
   +parentThreadID: ?string,
   +hasContainingThreadID: boolean,
   +rolePermissions: ?ThreadRolePermissionsBlob,
@@ -366,7 +366,7 @@
     return {
       roleColumnValue: role.toString(),
       depth: row.depth,
-      threadType: assertThreadType(row.type),
+      threadType: assertThinThreadType(row.type),
       parentThreadID: row.parent_thread_id
         ? row.parent_thread_id.toString()
         : null,
@@ -389,7 +389,7 @@
     return {
       roleColumnValue: role,
       depth: row.depth,
-      threadType: assertThreadType(row.type),
+      threadType: assertThinThreadType(row.type),
       parentThreadID: row.parent_thread_id
         ? row.parent_thread_id.toString()
         : null,
@@ -413,7 +413,7 @@
     return {
       roleColumnValue: row.id.toString(),
       depth: row.depth,
-      threadType: assertThreadType(row.type),
+      threadType: assertThinThreadType(row.type),
       parentThreadID: row.parent_thread_id
         ? row.parent_thread_id.toString()
         : null,
@@ -591,7 +591,7 @@
   +threadID: string,
   +parentThreadID: string,
   +containingThreadID: string,
-  +threadType: ThreadType,
+  +threadType: ThinThreadType,
   +depth: number,
   +users: Map<string, DescendantUserInfo>,
 };
@@ -645,7 +645,7 @@
         threadID: descendantThreadID,
         parentThreadID: row.parent_thread_id.toString(),
         containingThreadID: row.containing_thread_id.toString(),
-        threadType: assertThreadType(row.type),
+        threadType: assertThinThreadType(row.type),
         depth: row.depth,
         users: new Map(),
       });
@@ -776,7 +776,7 @@
     throw new ServerError('internal_error');
   }
   const [threadResult] = threadResults;
-  const threadType = assertThreadType(threadResult.type);
+  const threadType = assertThinThreadType(threadResult.type);
   const depth = threadResult.depth;
   const hasContainingThreadID = threadResult.containing_thread_id !== null;
   const parentThreadID = threadResult.parent_thread_id?.toString();
@@ -1376,7 +1376,7 @@
   const allThreads = result.map(row => {
     return {
       id: row.id.toString(),
-      type: assertThreadType(row.type),
+      type: assertThinThreadType(row.type),
       depth: row.depth,
     };
   });
diff --git a/lib/types/thread-types-enum.js b/lib/types/thread-types-enum.js
--- a/lib/types/thread-types-enum.js
+++ b/lib/types/thread-types-enum.js
@@ -60,7 +60,7 @@
   ...thickThreadTypes,
 });
 
-export function assertThreadType(threadType: number): ThreadType {
+export function assertThinThreadType(threadType: number): ThinThreadType {
   invariant(
     threadType === 3 ||
       threadType === 4 ||
@@ -72,10 +72,43 @@
       threadType === 10 ||
       threadType === 11 ||
       threadType === 12,
+    'number is not ThinThreadType enum',
+  );
+  return threadType;
+}
+
+export function assertThickThreadType(threadType: number): ThickThreadType {
+  invariant(
+    threadType === 13 ||
+      threadType === 14 ||
+      threadType === 15 ||
+      threadType === 16,
+    'number is not ThickThreadType enum',
+  );
+  return threadType;
+}
+
+export function assertThreadType(threadType: number): ThreadType {
+  invariant(
+    threadType === 3 ||
+      threadType === 4 ||
+      threadType === 5 ||
+      threadType === 6 ||
+      threadType === 7 ||
+      threadType === 8 ||
+      threadType === 9 ||
+      threadType === 10 ||
+      threadType === 11 ||
+      threadType === 12 ||
+      threadType === 13 ||
+      threadType === 14 ||
+      threadType === 15 ||
+      threadType === 16,
     'number is not ThreadType enum',
   );
   return threadType;
 }
+
 export const threadTypeValidator: TRefinement<number> = tNumEnum(
   values(threadTypes),
 );