diff --git a/src/calibre/srv/render_book.py b/src/calibre/srv/render_book.py
index 54acfdeff3..807312a234 100644
--- a/src/calibre/srv/render_book.py
+++ b/src/calibre/srv/render_book.py
@@ -395,7 +395,7 @@ def transform_html(container, name, virtualize_resources, link_uid, link_to_map,
             href = a.get(attr)
             if href:
                 href = link_replacer(name, href)
-            else:
+            elif attr in a.attrib:
                 a.set(attr, 'javascript:void(0)')
             if href and href.startswith(link_uid):
                 a.set(attr, 'javascript:void(0)')
@@ -535,7 +535,7 @@ def virtualize_html(container, name, link_uid, link_to_map, virtualized_names):
         elif href:
             a.set('target', '_blank')
             a.set('rel', 'noopener noreferrer')
-        else:
+        elif attr in a.attrib:
             a.set(attr, 'javascript:void(0)')
 
     for a in link_xpath(root):