diff --git a/zones/Makefile b/zones/Makefile
index f81a466..552a487 100644
--- a/zones/Makefile
+++ b/zones/Makefile
@@ -19,6 +19,9 @@ tzdb: download
 	@echo "Unpacking Olson TZDB release..."
 	tar -C code -xzf tzdata*tar.gz
 	tar -C code -xzf tzcode*tar.gz
+	cp bias2021a.bp code
+	! grep BACKPICK code/Makefile >/dev/null || \
+	  make -C code BACKPICK=bias2021a.bp PACKRATDATA=backzone tzdata.zi
 
 code/zone.tab: tzdb
 	@make -C code