diff --git a/lib/suo/client/redis.rb b/lib/suo/client/redis.rb
index 5f318d1..ad5d3b8 100644
--- a/lib/suo/client/redis.rb
+++ b/lib/suo/client/redis.rb
@@ -21,7 +21,7 @@ def set(key, newval, _)
           multi.set(key, newval)
         end
 
-        ret[0] == "OK"
+        ret && ret[0] == "OK"
       end
 
       def synchronize(key)