When passing a non-existent &binvar to $bvar().text, mIRC throws an error.
I believe it should return null empty text instead, since the parameters are not invalid.

//echo -a $bvar(&nonexist,1-).text
> * Invalid parameters: $bvar

To prove that the parameters are indeed not invalid, this similar command works.

//echo -a $bvar(&nonexist,0)
> 0


Well. At least I won lunch.
Good philosophy, see good in bad, I like!