Cool but it tends to crash a lot

I have had a lot of fun with this game and I have the final upgrade for the sentry guns and the solders but when ever I try to purchase the final upgrade for the shock tower with the sas Bucks my game freezes if you guys could fix that, that would be amazing.
p.s. The same thing happens with the sniper

burgur spider about SAS: Zombie Assault TD, v1.1