diff --git a/.travis.yml b/.travis.yml
index c1561a8..0d080c4 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -11,6 +11,8 @@ before_install:
   - Rscript -e 'install.packages("munsell", repos = "http://cran.rstudio.com")'
   - Rscript -e 'install.packages("scales", repos = "http://cran.rstudio.com")'
   - Rscript -e 'install.packages("ggplot2", repos = "http://cran.rstudio.com")'
+  - Rscript -e 'install.packages("knitr", repos = "http://cran.rstudio.com")'
+
 env:
  global:
    - CRAN: http://cran.rstudio.com