Hello!
I cannot suppose how you made this mistake, because letter *a* is disjunctive conjunction, not preposition. It's like "I am sleeping,*while* the dog eats my notebook."
The preposition is *o* for the sentence, it means *about*.
If you said this sentense aloud, nobody would notice a difference, because it sounds similarly.
I hope this explanation is not thoroughly too detailed, for I'm not ready to be "captain obvious". =D
Good luck!