I have run some quick tests and agree with @Airy's findings. Page loading in the forum is impossibly slow for me today (an unusual but not unprecedented problem) and also I am short of time – but I'll mention the following just in case they might contain some clues, and wish you luck! 
- Bug Report: Keyboard Maestro Does NOT Error if Macro Does NOT Exist in AppleScript
- Does This Variable Exist or Not?
- [the page won't load for me for this one!
] Variable apparently doesn't exist after `Set Variable To`, but can be shown in Alerts existsanddoes not existare not listed in the conditions forswitchandif/thenin the Wiki.