diff --git a/reference/Makefile b/reference/Makefile
index 133596a..73f4c35 100644
--- a/reference/Makefile
+++ b/reference/Makefile
@@ -1,2 +1,2 @@
 intptdac: intptdac.f
-	gfortran -o intptdac intptdac.f
+	gfortran -std legacy -o intptdac intptdac.f