diff --git a/docs/docsrc/Makefile b/docs/docsrc/Makefile
index 259a498..59ac25a 100644
--- a/docs/docsrc/Makefile
+++ b/docs/docsrc/Makefile
@@ -4,7 +4,7 @@
 # You can set these variables from the command line, and also
 # from the environment for the first two.
 SPHINXOPTS    +=
-SPHINXBUILD   ?= sphinx-build
+SPHINXBUILD   += sphinx-build
 SOURCEDIR     = source
 BUILDDIR      = build