How to provide user defined function for python blaze with sqlite backend?

I connect to sqlite database in Blaze using df = bz.Data("sqlite:///<mydatabase>) everything works fine but I do not know how to provide user-defined functions in my interaction with df. I have a column called IP in df which is text containing IP addresses. I also have a function toSubnet (x, y) which takes in an IP address (x) in text format and return its /y subnet. For example:

out = toSubnet('1.1.1.1',24)
out
1.1.1.0/24

Now if I want to map all IPs to their /14 subnets, I use:

df.IP.map(lambda x:toSubnet(x,14),'string')

Which works when the backend is CSV. But with sqlite backend I get NotImplementedError. What's wrong here?