diff --git a/usr.bin/units/units.c b/usr.bin/units/units.c
index c2141f6c934..70bd9ec5b63 100644
--- a/usr.bin/units/units.c
+++ b/usr.bin/units/units.c
@@ -245,7 +245,7 @@ addsubunit(char *product[], char *toadd)
 		return 1;
 	}
 	if (!*ptr)
-		*(ptr + 1) = 0;
+		*(ptr + 1) = NULL;
 	*ptr = dupstr(toadd);
 	return 0;
 }