diff --git a/src/calibre/ebooks/oeb/base.py b/src/calibre/ebooks/oeb/base.py
index a77554062e..69b2d67b7a 100644
--- a/src/calibre/ebooks/oeb/base.py
+++ b/src/calibre/ebooks/oeb/base.py
@@ -1181,12 +1181,6 @@ class Manifest(object):
         :param:`href` are `None` then the corresponding item in the return
         tuple will also be `None`.
         """
-        def href_present(href):
-            for x in self.hrefs:
-                if x.lower() == href.lower():
-                    return True
-            return False
-
         if id is not None:
             base = id
             index = 1
@@ -1197,7 +1191,8 @@ class Manifest(object):
             href = urlnormalize(href)
             base, ext = os.path.splitext(href)
             index = 1
-            while href_present(href):
+            lhrefs = set([x.lower() for x in self.hrefs])
+            while href.lower() in lhrefs:
                 href = base + str(index) + ext
                 index += 1
         return id, href