diff --git a/lib/spack/docs/Makefile b/lib/spack/docs/Makefile
index 95d26041b7..bdbdab7e8b 100644
--- a/lib/spack/docs/Makefile
+++ b/lib/spack/docs/Makefile
@@ -7,7 +7,7 @@ SPHINXBUILD   = sphinx-build
 PAPER         =
 BUILDDIR      = _build
 
-export PYTHONPATH = ../../spack
+export PYTHONPATH := ../../spack:$(PYTHONPATH)
 APIDOC_FILES  = spack*.rst
 
 # Internal variables.