diff --git a/src/calibre/srv/render_book.py b/src/calibre/srv/render_book.py
index c4a4466ff1..e9c30678d1 100644
--- a/src/calibre/srv/render_book.py
+++ b/src/calibre/srv/render_book.py
@@ -443,6 +443,9 @@ def map_epub_type(epub_type, attribs, elem):
             attribs[i] = ['role', role]
 
 
+known_tags = ('img', 'script', 'link', 'image', 'style')
+
+
 def serialize_elem(elem, nsmap):
     ns, name = split_name(elem.tag)
     nl = name.lower()
@@ -450,7 +453,7 @@ def serialize_elem(elem, nsmap):
         ns, name = None, 'epub-' + name
     if nl == 'meta':
         return  # Filter out <meta> tags as they have unknown side-effects
-    if nl in {'img', 'script', 'link', 'image', 'style'}:
+    if nl in known_tags:
         name = nl
     ans = {'n':name}
     if elem.text:
diff --git a/src/pyj/read_book/resources.pyj b/src/pyj/read_book/resources.pyj
index efb35bf5d2..4c5ea4a14e 100644
--- a/src/pyj/read_book/resources.pyj
+++ b/src/pyj/read_book/resources.pyj
@@ -164,7 +164,7 @@ def finalize_resources(book, root_name, resource_data):
 
     return root_data, mathjax, blob_url_map
 
-js_types = set('text/javascript text/ecmascript application/javascript application/ecmascript'.split(' '))
+js_types = {k: True for k in 'text/javascript text/ecmascript application/javascript application/ecmascript'.split(' ')}
 resource_tag_names = {'script':'src', 'link':'href', 'img':'src', 'image':'xlink:href'}
 ns_rmap = {'http://www.w3.org/2000/svg':'svg', 'http://www.w3.org/1999/xlink':'xlink', 'http://www.w3.org/1998/Math/MathML':'math', 'http://www.w3.org/XML/1998/namespace': 'xml'}
 ns_count = 0
@@ -201,11 +201,12 @@ def process_stack(stack, tag_map, ns_map, load_required, onload, resource_urls):
         if attr:
             if attr.indexOf(':') != -1:
                 attr = attr.replace('xlink:', '')
-            for a in (src.a or v'[]'):
-                if a[0] is attr:
-                    loadable = a[1].startswith('blob:')
-                    resource_urls[a[1]] = True
-                    break
+            if src.a:
+                for a in src.a:
+                    if a[0] is attr:
+                        loadable = a[1].startswith('blob:')
+                        resource_urls[a[1]] = True
+                        break
         if loadable:
             load_required.add(node[0])
             elem.addEventListener('load', onload.bind(node[0]))
@@ -214,7 +215,7 @@ def process_stack(stack, tag_map, ns_map, load_required, onload, resource_urls):
         apply_attributes(src, elem, ns_map)
         parent.appendChild(elem)
         if src.x:
-            if src.n is 'script' and (elem.getAttribute('type') or 'text/javascript').toLowerCase() in js_types:
+            if src.n is 'script' and js_types[(elem.getAttribute('type') or 'text/javascript').toLowerCase()] is True:
                 elem.text = src.x
             else:
                 elem.appendChild(document.createTextNode(src.x))