Preparing for the 3rd Introduction to R slides

I am now preparing the slides for the 3rd Introduction to R Mini Workshop. I am using R Markdown to write the slides. Just found out that somehow if I used the "smaller: yes" or "smaller: true" line in the YAML lines (see the following lines),

---
title: "Introduction to R: R in the library"
author: "Dasapta Erwin Irawan"
date: "26/05/2015"
output:
  ioslides_presentation:
    logo: itb.jpg
    smaller: yes
    widescreen: yes
    self_contained: no
---

I ended up getting the following errors.

pandoc: user error ([string "custom"]:253: attempt to concatenate field 'class' (a nil value))

Error: pandoc document conversion failed with error 1Execution halted

The error was dismissed if I delete the above-mentioned line.