diff --git a/monkey_island/cc/resources/pthmap.py b/monkey_island/cc/resources/pthmap.py
index ade484ea7..59fa15832 100644
--- a/monkey_island/cc/resources/pthmap.py
+++ b/monkey_island/cc/resources/pthmap.py
@@ -826,7 +826,7 @@ class PassTheHashMap(object):
 
     @cache
     def GetNonCritialServers(self):
-        return self.machines - self.GetCritialServers()
+        return set(self.machines) - self.GetCritialServers()
         
     @cache
     def GetThreateningUsersByVictim(self, victim):