Creating a security group without a source returns a generic exception

[BennieCopeland]

Attempting to create a security rule without providing a source returns an unhelpful Unhandled exception. System.ArgumentException: The input sequence was empty.

let httpAccess =
    securityRule {
        name "accept-web"
        services [
            NetworkService ("https", Port 443us)
            NetworkService ("http", Port 80us)
        ]
    }

let myNsg =
    nsg {
        name "my-nsg"
        add_rules [
            httpAccess
        ]
    }