diff --git a/lib/diskcached.rb b/lib/diskcached.rb
index 263fb52..20a6d6b 100644
--- a/lib/diskcached.rb
+++ b/lib/diskcached.rb
@@ -45,11 +45,10 @@ def timeout=(t)
     @gc_time = t
   end
 
-  # return true if cache with 'key' is expired
+  # return true if cache with 'key' is expired or there is no cache entry
   def expired? key
-    return false if timeout.nil?
     mtime = read_cache_mtime(key)
-    return (mtime.nil? || mtime+timeout <= Time.now)
+    return mtime.nil? || (timeout && mtime+timeout <= Time.now)
   end
 
   # expire cache with 'key'