Can’t scan with ClamXav on my Mac

I recently downloaded and installed ClamXav at my macbook.
The problem is that I can't use it at all. Everytime I try to manually scan a folder or file, I get the following error message

Sorry, scanning the entire startup disk causes problems and is not allowed.
You can, however, select multiple items by holding down the command key while clicking files/folders
Oops!
This is really strange, because I am not trying to scan the entire disk, but only a folder.
I tried to uninstall and install again, but the problem persists.
