It shouldn't be accepted. The noun 'lightning' is an uncountable noun. Hence, you cannot use articles (a, the) with it.

If you're thinking "But wait, we can count how many times lightning strikes". Yes, if you want to use it in plural form, you say something like "I saw three lightning strikes" instead of "I saw three lightnings".

I think I'll never get it right, too -:(

+100