How do you get the cupid zombie in zombie farm?

Whoever said buy it at level 25 is wrong because I am at level 31 and it is still grayed out. I bought the heart gravestone and I still can't buy it. I tried combining a girl and a regular zombie and it doesn't work so I don't know how to get it.

***You have to complete all of the V-Day Challenges to get A cupid zombie. And yes, just on zombie per 12 brains and lots of coins.