diff --git a/mopidy/core/actor.py b/mopidy/core/actor.py
index f20b0ba2..6d10a593 100644
--- a/mopidy/core/actor.py
+++ b/mopidy/core/actor.py
@@ -161,7 +161,7 @@ class Backends(list):
 
             for scheme in b.uri_schemes.get():
                 assert scheme not in backends_by_scheme, (
-                    'Cannot add URI scheme %s for %s, '
+                    'Cannot add URI scheme "%s" for %s, '
                     'it is already handled by %s'
                 ) % (scheme, name(b), name(backends_by_scheme[scheme]))
                 backends_by_scheme[scheme] = b