line 133: WARNING: unbound variable

When I attempt to run the script as shown in the README.md I get the following:

```
hp-mint al # curl -s https://raw.githubusercontent.com/xdissent/ievms/master/ievms.sh | bash
Checking for VirtualBox
Checking for Oracle VM VirtualBox Extension Pack
bash: line 133: WARNING: unbound variable
hp-mint al # 
```

I am running the latest stable version of Mint under Xfce
