diff --git a/doc/Makefile b/doc/Makefile
index 683e4b5609..050d9dd239 100644
--- a/doc/Makefile
+++ b/doc/Makefile
@@ -6,7 +6,7 @@ subdir-y :=
 
 # You can set these variables from the command line.
 SPHINXBUILD   = sphinx-build
-SPHINXOPTS    =
+SPHINXOPTS    = -W
 SPHINXDIRS    = .
 _SPHINXDIRS   = $(patsubst $(srctree)/doc/%/conf.py,%,$(wildcard $(srctree)/doc/*/conf.py))
 SPHINX_CONF   = conf.py