When this bash script is run *backgrounded*, it terminates the calling shell. Why is this?

```
#!/bin/bash
#
# weird job control, background termination of parent shell problem
set -eum
echo "Do not go gentle into that good night"
sleep 1

```

Save this as `wat`, `chmod +x wat`, run it as `wat` - no problem. Run it *backgrounded*:

```
wat &

```

It closes my current shell!? And even more strangely, if that last sleep line is commented out, it can run backgrounded quite happily. Can anyone explain?

I understand that it's to do with the `-m` (job control) option to `set` but I don't understand why...