In our tests we have noticed that passing a large number of variable arguments (in our case, more than 70) to a Lua function can sometimes result in memory corruption. If batch_get() is being called with a large number of arguments, this might be the source of the crashes.
You might try changing batch_get() to take only three parameters, with the third parameter being a list of keys. This approach has worked well for us.
Hope this helps,
Brad