diff --git a/src/vim.h b/src/vim.h
index 804070e..13f00ce 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -52,7 +52,9 @@
 
 /* user ID of root is usually zero, but not for everybody */
 #ifdef __TANDEM
-# define _TANDEM_SOURCE
+# ifndef _TANDEM_SOURCE
+#  define _TANDEM_SOURCE
+# endif
 # include <floss.h>
 # define ROOT_UID 65535
 #else