"This class is not key value coding-compliant for the key " empty key

Question

I'm getting this famous error

[viewController setValue:forUndefinedKey:] this class is not key value coding-compliant for the key .

but the problem is that as you can tell above, the key is empty (aka @""). if I put following code

- (void)setValue:(id)value forUndefinedKey:(NSString *)key
{
    NSLog(@"UndefinedKey is: %@", key);
}

in console I get

UndefinedKey is:

Putting a breakpoint and inspecting the key says it's empty.