diff --git a/GNUmakefile.in b/GNUmakefile.in
index 30553b2a95..27357e5e3b 100644
--- a/GNUmakefile.in
+++ b/GNUmakefile.in
@@ -102,10 +102,18 @@ distdir-location:
 # on, Unix machines.
 
 $(distdir).tar.gz:
-	$(GIT) -C $(srcdir) -c core.autocrlf=false archive --format tar.gz -9 --prefix $(distdir)/ HEAD -o $(abs_top_builddir)/$@
+ifneq ($(PG_COMMIT_REFSPEC),)
+	$(GIT) -C $(srcdir) -c core.autocrlf=false archive --format tar.gz -9 --prefix $(distdir)/ $(PG_COMMIT_REFSPEC) -o $(abs_top_builddir)/$@
+else
+	@echo "Please specify PG_COMMIT_REFSPEC." && exit 1
+endif
 
 $(distdir).tar.bz2:
-	$(GIT) -C $(srcdir) -c core.autocrlf=false -c tar.tar.bz2.command='$(BZIP2) -c' archive --format tar.bz2 --prefix $(distdir)/ HEAD -o $(abs_top_builddir)/$@
+ifneq ($(PG_COMMIT_REFSPEC),)
+	$(GIT) -C $(srcdir) -c core.autocrlf=false -c tar.tar.bz2.command='$(BZIP2) -c' archive --format tar.bz2 --prefix $(distdir)/ $(PG_COMMIT_REFSPEC) -o $(abs_top_builddir)/$@
+else
+	@echo "Please specify PG_COMMIT_REFSPEC." && exit 1
+endif
 
 distcheck: dist
 	rm -rf $(dummy)