Hoolio is correct, we currently do not support IPv6 traffic in ASM. I am sure that something is choking somewhere there when ASM tries to look at the IPv6 address.
Until ASM has full native IPv6 support, there might be a way to accomplish what you want using layered virtual servers. Essentially, you would have an IPv6 Virtual server that would do SNAT and target the internal IPv4 virtual server that will have ASM enabled. Since ASM will see the source IP as v4(after translation), things should work without a hitch - and that ASM virtual will then SNAT again and load-balance to the pool of actual IPv4 servers. The downside of this approach is that you will lose your source IP address in the ASM logs for violations.