>would it be better to give an error than to allow this?

That's no so simple. Actually I was already about giving up a complete evaluation of a variable in a property when I saw it grows more complicated but... But with a error detection it tends to become even more cryptic,
consider the following code:
```
@p: ~' ' ~`1 + 1, "  "` boo;

z {
    @{p}: 0;
}
```
It's a bit exaggerated example of course but there's no way for the compiler to 100% decide if the property name is correct or it isn't unless it completely re-parsers the result string all over again. So my main goal for this fix was just to make sure a variable is expanded "as is", i.e. the same way it works with selector interpolation (or within backticks interpolation), i.e. as in (#1760):
>In general LESS never tests if its output is a valid CSS 

This way we don't have to invent any new special rules for each interpolation type (and a user at least knows what to expect there w/o any special documentation). And this test is only to ensure the expansion works properly.