diff --git a/src/calibre/srv/handler.py b/src/calibre/srv/handler.py
index 98122960f5..e25c74f566 100644
--- a/src/calibre/srv/handler.py
+++ b/src/calibre/srv/handler.py
@@ -6,7 +6,7 @@ from __future__ import (unicode_literals, division, absolute_import,
 __license__ = 'GPL v3'
 __copyright__ = '2015, Kovid Goyal <kovid at kovidgoyal.net>'
 
-import os
+import os, json
 from collections import OrderedDict
 from importlib import import_module
 from threading import Lock
@@ -137,7 +137,10 @@ class Context(object):
             old = cache.pop(key, None)
             if old is None or old[0] <= db.last_modified():
                 categories = db.get_categories(book_ids=restrict_to_ids, sort=opts.sort_by, first_letter_sort=opts.collapse_model == 'first letter')
-                cache[key] = old = (utcnow(), render(db, categories))
+                data = json.dumps(render(db, categories), ensure_ascii=False)
+                if isinstance(data, type('')):
+                    data = data.encode('utf-8')
+                cache[key] = old = (utcnow(), data)
                 if len(cache) > self.CATEGORY_CACHE_SIZE:
                     cache.popitem(last=False)
             else:
diff --git a/src/calibre/srv/routes.py b/src/calibre/srv/routes.py
index d323d216f4..66d482b47b 100644
--- a/src/calibre/srv/routes.py
+++ b/src/calibre/srv/routes.py
@@ -18,9 +18,12 @@ default_methods = frozenset(('HEAD', 'GET'))
 
 def json(ctx, rd, endpoint, output):
     rd.outheaders['Content-Type'] = 'application/json; charset=UTF-8'
-    ans = jsonlib.dumps(output, ensure_ascii=False)
-    if not isinstance(ans, bytes):
-        ans = ans.encode('utf-8')
+    if isinstance(output, bytes):
+        ans = output  # Assume output is already UTF-8 encoded json
+    else:
+        ans = jsonlib.dumps(output, ensure_ascii=False)
+        if not isinstance(ans, bytes):
+            ans = ans.encode('utf-8')
     return ans
 
 def route_key(route):