diff --git a/Makefile b/Makefile
index dddea44..fdb9401 100644
--- a/Makefile
+++ b/Makefile
@@ -30,8 +30,8 @@ all:
 PREFIX = /usr/local
 
 install: all
-	install src/libpcg_random.a $PREFIX/lib
-	install -m 0644 include/pcg_variants.h $PREFIX/include
+	install src/libpcg_random.a ${PREFIX}/lib
+	install -m 0644 include/pcg_variants.h ${PREFIX}/include
 
 test:   all
 	cd test-low; $(MAKE) test