Why is the Reference section not numbered when I use number_sections: true with R Markdown for PDF output?

I'm trying to generate a PDF document with numbered sections, citations, and a bibliography from R Markdown using rmarkdown::render. It works fine except that the References section is not numbered.

Here's a minimal working example:

---
output:
  pdf_document: 
    number_sections: true
references:
- id: fenner2012a
  title: One-click science marketing
  author:
  - family: Fenner
    given: Martin
---

# Introduction
See [@fenner2012a].

# References

I would expect rendering this to result in:

1. Introduction
See (Fenner 2012).

2. References
Fenner, Martin. n.d. “One-Click Science Marketing.”

But instead I get (note that References is missing a section number):

1. Introduction
See (Fenner 2012).

References
Fenner, Martin. n.d. “One-Click Science Marketing.”

Any ideas?