diff --git a/docs/Makefile b/docs/Makefile
index c710ff08..461b1e3a 100644
--- a/docs/Makefile
+++ b/docs/Makefile
@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line, and also
 # from the environment for the first two.
-SPHINXOPTS    ?=
+SPHINXOPTS    ?= -W
 SPHINXBUILD   ?= sphinx-build
 SOURCEDIR     = source
 BUILDDIR      = build