Which is correct?

Hello,
today I wrote English exams to my High school and right after that on the Internet appeared all aswers. Therefore, I have a question. Which version of this sentence is correct:
-You must come to visit me when I will decorate my room.
-You must come to visit me when I decorate my room.
I was sure that there should be used future simple, but according to not official answers "decorate" is correct. If it's correct, I would be grateful for explaining why.

Greetings

Selu.