diff --git a/IV/Makefile b/IV/Makefile
index 1fa567d..2b22e8b 100644
--- a/IV/Makefile
+++ b/IV/Makefile
@@ -63,9 +63,9 @@ slides-2021-03-29.html: slides-2021-03-29.txt
 $(SOURCE):
 	wget http://download.geofabrik.de/europe/$@
 
+REF = $(shell git describe --tags --dirty)
 version.tex: $(shell git rev-parse --show-toplevel)/.git
 	( \
 		date '+\gdef\GeneratedAt{%F %T %Z}%'; \
 		printf '\gdef\VCDescribe{%s}%%\n' $(REF); \
 	) > $@
-