Spotlight not indexing secondary drive

I have two drives on my MacBook Pro. One is a SSD with the root (/) partition, and the other is a hard drive with a home directory partition (/Users). Both partitions are HFS+ format.

When I make a search using Spotlight, nothing on the /Users partition is searchable, since it is not indexed.

Trying to use mdutil to start indexing the drive does not work:

## Attempt to enable indexing and flush any existing index
$ mdutil -i on -E /Users
/Users:
        Indexing and searching disabled.

## Attempting the same as above to all volumes
$ mdutil -i on -E -a
/:
        Indexing enabled. 
/Users:
        Indexing and searching disabled.