diff --git a/README.md b/README.md
index 553f05f..e8f2b00 100644
--- a/README.md
+++ b/README.md
@@ -104,7 +104,7 @@ Processors for each book format return an object that implements the following i
 - `.isExternal(href)`: returns a boolean. If `true`, the link should be opened externally.
 
 The following methods are consumed by `progress.js`, for getting the correct TOC and page list item when navigating:
-- `.splitTOCHref(href)`: given an href string (from the TOC), returns an array, the first element of which is the `id` of the section (see above), and the second element is the fragment identifier (can be any type; see below)
+- `.splitTOCHref(href)`: given an href string (from the TOC), returns an array, the first element of which is the `id` of the section (see above), and the second element is the fragment identifier (can be any type; see below). May be async.
 - `.getTOCFragment(doc, id)`: given a `Document` object and a fragment identifier (the one provided by `.splitTOCHref()`; see above), returns a `Node` representing the target linked by the TOC item
 
 Almost all of the properties and methods are optional. At minimum it needs `.sections` and the `.load()` method for the sections, as otherwise there won't be anything to render.
diff --git a/progress.js b/progress.js
index 55f918b..26961dc 100644
--- a/progress.js
+++ b/progress.js
@@ -16,12 +16,12 @@ const flatten = items => items
     .flat()
 
 export class TOCProgress {
-    constructor({ toc, ids, splitHref, getFragment }) {
+    async init({ toc, ids, splitHref, getFragment }) {
         assignIDs(toc)
         const items = flatten(toc)
         const grouped = new Map()
         for (const [i, item] of items.entries()) {
-            const [id, fragment] = splitHref(item?.href) ?? []
+            const [id, fragment] = await splitHref(item?.href) ?? []
             const value = { fragment, item }
             if (grouped.has(id)) grouped.get(id).items.push(value)
             else grouped.set(id, { prev: items[i - 1], items: [value] })
@@ -36,6 +36,7 @@ export class TOCProgress {
         this.getFragment = getFragment
     }
     getProgress(index, range) {
+        if (!this.ids) return
         const id = this.ids[index]
         const obj = this.map.get(id)
         if (!obj) return null
diff --git a/view.js b/view.js
index 4609ece..94cb8d5 100644
--- a/view.js
+++ b/view.js
@@ -91,7 +91,8 @@ export class View extends HTMLElement {
             this.#sectionProgress = new SectionProgress(book.sections, 1500, 1600)
             const splitHref = book.splitTOCHref.bind(book)
             const getFragment = book.getTOCFragment.bind(book)
-            this.#tocProgress = new TOCProgress({
+            this.#tocProgress = new TOCProgress()
+            await this.#tocProgress.init({
                 toc: book.toc ?? [], ids, splitHref, getFragment })
             this.#pageProgress = new TOCProgress({
                 toc: book.pageList ?? [], ids, splitHref, getFragment })