diff --git a/user-guide/Makefile b/user-guide/Makefile
index 5e2de8f..fd9db7e 100644
--- a/user-guide/Makefile
+++ b/user-guide/Makefile
@@ -153,7 +153,7 @@ doctest:
 	      "results in $(BUILDDIR)/doctest/output.txt."
 
 pdf:
-	TEMP_DIR=`python pdf_optimise.py`
+	TEMP_DIR := $(python pdf_optimise.py)
 	$(SPHINXBUILD) -b pdf $(ALLSPHINXOPTS) build/pdf
 	python pdf_optimise.py restore $(TEMP_DIR)
 	@echo