diff --git a/src/calibre/library/coloring.py b/src/calibre/library/coloring.py
index fc14ef1efd..26d709a321 100644
--- a/src/calibre/library/coloring.py
+++ b/src/calibre/library/coloring.py
@@ -127,7 +127,7 @@ class Rule(object): # {{{
                 val, lt, eq, gt)
 
     def multiple_condition(self, col, action, val, sep):
-        if sep == '|':
+        if not sep or sep == '|':
             sep = ','
         if action == 'is set':
             return "test(field('%s'), '1', '')"%col