Checking for `null` doesn't seem to match with the error message. For the non-empty check, we can use the [`ImmutableSet.Builder`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/ImmutableSet.Builder.html#add\(E\)):
```
ImmutableSet.Builder builder = ImmutableSet.builder();
for (String network : checkNotNull(networks, "networks")) {
  builder.add(emptyToNull(network)); // will NPE if network is null
}
this.networks = builder.build();
return this
```