Some keys get silently dropped from output

The example from the README works for me:

```scala
BuildInfoKey.map(name) { case (k, v) => "project" + k.capitalize -> v.capitalize }
```

but if I try e.g.:

```scala
BuildInfoKey.map(moduleID) { case (k, v) ⇒ "projectMod" -> v.name }
```

I get no errors or warnings, but there is no `projectMod` key/value in the output. 

Why might this be?