diff --git a/book/Makefile b/book/Makefile
index 15c0f8a..a8776a5 100644
--- a/book/Makefile
+++ b/book/Makefile
@@ -6,7 +6,7 @@
 
 # You can set these variables from the command line, and also
 # from the environment for the first two.
-SPHINXOPTS    ?=
+SPHINXOPTS    ?= -W
 SPHINXBUILD   ?= sphinx-build
 LYCHEE ?= lychee
 SOURCEDIR     = source