diff --git a/vignettes/PimentelGomes.Rmd b/vignettes/PimentelGomes.Rmd
index 5a50d0e442980e85444e07b41b18878d179c8f4e..672d8b62daf391577b778926a09dfc00118524a6 100644
--- a/vignettes/PimentelGomes.Rmd
+++ b/vignettes/PimentelGomes.Rmd
@@ -77,6 +77,8 @@ img {
 </style>
 
 ```{r setup, include=FALSE}
+library(knitr)
+
 opts_chunk$set(
     dev.args=list(family="Palatino"))