diff --git a/lib/spack/docs/Makefile b/lib/spack/docs/Makefile
index 1ce5c1910c..1a8c13e8fb 100644
--- a/lib/spack/docs/Makefile
+++ b/lib/spack/docs/Makefile
@@ -2,7 +2,7 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    = -E
+SPHINXOPTS    = -W
 SPHINXBUILD   = sphinx-build
 PAPER         =
 BUILDDIR      = _build