Help! Can't make glossary using taxonomy term name

I'm using Drupal 8.0.2 and I'm trying to create a glossary of taxonomy terms by name. I have setup a simple view as follows:

• Unformatted list
• Fields: Taxonomy term: Name
• Filter: Taxonomy term: Vocabulary (= Fruit Name)
• Sort: Taxonomy term: Name (asc)

Stopping here the preview shows all Fruits by name (a-z) as expected.

The problem is when I add the following Contextual Filter:

• Taxonomy term: Name
• Type: Fixed
• Fixed value: a
• More: Glossary mode
• Character limit: 1

The preview is empty, and the page returns a 500 error. I have done something similar many times using Content: Title, but it never works using Taxonomy: Name. Can anyone tell me why this won't work using taxonomies?