diff --git a/app/common/UserAPI.ts b/app/common/UserAPI.ts
index 08f9df80..aa84ef21 100644
--- a/app/common/UserAPI.ts
+++ b/app/common/UserAPI.ts
@@ -773,10 +773,10 @@ export class UserAPIImpl extends BaseAPI implements UserAPI {
   }
 
   public async getWorker(key: string): Promise<string> {
-    const json = await this.requestJson(`${this._url}/api/worker/${key}`, {
+    const json: PublicDocWorkerUrlInfo = (await this.requestJson(`${this._url}/api/worker/${key}`, {
       method: 'GET',
       credentials: 'include'
-    });
+    })) as PublicDocWorkerUrlInfo;
     return getPublicDocWorkerUrl(this._homeUrl, json);
   }
 
@@ -1168,7 +1168,7 @@ export type PublicDocWorkerUrlInfo = {
   docWorkerUrl: null;
 } | {
   selfPrefix: null;
-  docWorkerUrl: string|null;
+  docWorkerUrl: string;
 }
 
 export function getUrlFromPrefix(homeUrl: string, prefix: string) {
@@ -1191,6 +1191,7 @@ export function getUrlFromPrefix(homeUrl: string, prefix: string) {
  *                               (result of the call to /api/worker/:docId)
  */
 export function getPublicDocWorkerUrl(homeUrl: string, docWorkerInfo: PublicDocWorkerUrlInfo) {
-  const publicUrl = docWorkerInfo.docWorkerUrl;
-  return publicUrl || getUrlFromPrefix(homeUrl, docWorkerInfo?.selfPrefix);
+  return docWorkerInfo.selfPrefix ?
+    getUrlFromPrefix(homeUrl, docWorkerInfo.selfPrefix) :
+    docWorkerInfo.docWorkerUrl!;
 }
diff --git a/app/server/lib/AppEndpoint.ts b/app/server/lib/AppEndpoint.ts
index 33f38a4e..a676f5e7 100644
--- a/app/server/lib/AppEndpoint.ts
+++ b/app/server/lib/AppEndpoint.ts
@@ -195,10 +195,16 @@ export function attachAppEndpoint(options: AttachOptions): void {
       });
     }
 
+    // When no doc worker seed, we're in single server mode.
+    // Return null, to signify that the URL prefix serving the
+    // current endpoint is the only one available.
+    const publicUrl = docStatus?.docWorker?.publicUrl;
+    const workerPublicUrl = publicUrl !== undefined ? customizeDocWorkerUrl(publicUrl, req) : null;
+
     await sendAppPage(req, res, {path: "", content: body.page, tag: body.tag, status: 200,
                                  googleTagManager: 'anon', config: {
       assignmentId: docId,
-      getWorker: {[docId]: customizeDocWorkerUrl(docStatus?.docWorker?.publicUrl, req)},
+      getWorker: {[docId]: workerPublicUrl },
       getDoc: {[docId]: pruneAPIResult(doc as unknown as APIDocument)},
       plugins
     }});
diff --git a/app/server/lib/DocWorkerUtils.ts b/app/server/lib/DocWorkerUtils.ts
index 25be2234..a5bc6cf9 100644
--- a/app/server/lib/DocWorkerUtils.ts
+++ b/app/server/lib/DocWorkerUtils.ts
@@ -36,16 +36,7 @@ import { getAssignmentId } from './idUtils';
  * TODO: doc worker registration could be redesigned to remove the assumption
  * of a fixed base domain.
  */
-export function customizeDocWorkerUrl(
-  docWorkerUrlSeed: string|undefined,
-  req: express.Request
-): string|null {
-  if (!docWorkerUrlSeed) {
-    // When no doc worker seed, we're in single server mode.
-    // Return null, to signify that the URL prefix serving the
-    // current endpoint is the only one available.
-    return null;
-  }
+export function customizeDocWorkerUrl( docWorkerUrlSeed: string, req: express.Request): string {
   const docWorkerUrl = new URL(docWorkerUrlSeed);
   const workerSubdomain = parseSubdomainStrictly(docWorkerUrl.hostname).org;
   adaptServerUrl(docWorkerUrl, req);