diff --git a/manual/Makefile b/manual/Makefile
index b516693..9f5ded6 100644
--- a/manual/Makefile
+++ b/manual/Makefile
@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line.
 SPHINXOPTS    =
-SPHINXBUILD   = `which sphinx-build-3 || which sphinx-build`
+SPHINXBUILD = $(shell which sphinx-build-3 || which sphinx-build)
 PAPER         =
 
 # Internal variables.