...

src/pyj/read_book/resources.pyj

@@ -27,13 +27,13 @@ def load_resources(book, root_name, previous_resources, proceed):
     link_pat = create_link_pat(book)
 
     def do_one():
-        if not pending_resources.length:
+        name = pending_resources.shift()
+        if not name:
             for k in previous_resources:
                 v'delete previous_resources[k]'
             if book.manifest.files[root_name].has_maths:
                 return load_mathjax(book, ans, proceed)
             return proceed(ans)
-        name = pending_resources.shift()
         if ans[name]:
             return setTimeout(do_one, 0)
         if previous_resources[name]:

src/pyj/viewer-main.pyj

@@ -54,6 +54,8 @@ def file_received(name, file_data, proceed, end_type, xhr, ev):
 
 def get_file(book, name, proceed):
     entry = book.manifest.files[name]
+    if not entry:
+        raise ValueError(f'No file named {name} in the book manifest')
     xhr = ajax('book/' + name, file_received.bind(None, name, entry, proceed), ok_code=0)
     if entry.is_html or entry.mimetype.startswith('text/') or entry.mimetype is 'application/javascript':
         xhr.responseType = 'text'