I've constructed a lava moat to help defend my house after entering Hardmode. 

![](https://i.stack.imgur.com/A7FmK.png)

Unfortunately it seems like NPCs that are spawning and falling into the moat. I'm getting messages like: 

> 
> Jake the Guide has arrived!  
> 
>  Jake the Guide was slain! 
> 
> 
> 

How can I stop them from committing suicide in my moat?