diff --git a/Makefile b/Makefile
index 6c2d287..a2f5caa 100644
--- a/Makefile
+++ b/Makefile
@@ -20,7 +20,7 @@ install:
 	@mkdir -p $(DESTDIR)$(BINPREFIX)
 	@echo "... installing bins to $(DESTDIR)$(BINPREFIX)"
 	@echo "... installing man pages to $(DESTDIR)$(MANPREFIX)"
-	$(eval TEMPFILE := $(shell mktemp -q ${TMPDIR:-/tmp}/git-extras.XXXXXX 2>/dev/null || mktemp -q))
+	$(eval TEMPFILE := $(shell mktemp -q $${TMPDIR:-/tmp}/git-extras.XXXXXX 2>/dev/null || mktemp -q))
 	@# chmod from rw-------(default) to rwxrwxr-x, so that users can exec the scripts
 	@chmod 775 $(TEMPFILE)
 	$(eval EXISTED_ALIASES := $(shell \