diff --git a/doc/Makefile b/doc/Makefile
index 62ef91ba813..de5921267ad 100644
--- a/doc/Makefile
+++ b/doc/Makefile
@@ -2,7 +2,7 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    =
+SPHINXOPTS    ?=
 SPHINXBUILD   = sphinx-build
 PAPER         =
 BUILDDIR      = build