diff --git a/src/lib/data/catalogData.ts b/src/lib/data/catalogData.ts
index b4bd2cd0a..c66189d21 100644
--- a/src/lib/data/catalogData.ts
+++ b/src/lib/data/catalogData.ts
@@ -23,7 +23,7 @@ export interface CatalogData {
     }[];
     tags: {};
     quizzes: {};
-    htmlBooks: {
+    htmlBooks?: {
         id: string;
         name: string;
     }[];
diff --git a/src/lib/data/navigation.ts b/src/lib/data/navigation.ts
index a293b2a4c..15f8cbd7f 100644
--- a/src/lib/data/navigation.ts
+++ b/src/lib/data/navigation.ts
@@ -74,7 +74,10 @@ export class NavigationContext {
             this.collection = docSet.split('_')[1];
             this.catalog = await this.fetchCatalog(this.docSet);
             this.books = this.catalog.documents.map((b) => b.bookCode);
-            this.allBookIds = [...this.books, ...this.catalog.htmlBooks.map((b) => b.id)];
+            this.allBookIds = [
+                ...this.books,
+                ...(this.catalog.htmlBooks ? this.catalog.htmlBooks.map((b) => b.id) : [])
+            ];
             newBook = true;
         }
         if (book !== this.book && this.allBookIds.includes(book)) {
@@ -128,7 +131,7 @@ export class NavigationContext {
             return;
         }
 
-        const htmlBook = this.catalog.htmlBooks.find((b) => b.id === this.book);
+        const htmlBook = this.catalog.htmlBooks?.find((b) => b.id === this.book);
         if (htmlBook) {
             this.title = htmlBook.name;
             this.name = htmlBook.name;