diff --git a/crypto/bn/bn.h b/crypto/bn/bn.h
index 573666769f..103a1bdd09 100644
--- a/crypto/bn/bn.h
+++ b/crypto/bn/bn.h
@@ -89,8 +89,9 @@ extern "C" {
  * using "long long's", are 32bit, and are not using my assembler code. */
 #if defined(OPENSSL_SYS_MSDOS) || defined(OPENSSL_SYS_WINDOWS) || \
     defined(OPENSSL_SYS_WIN32) || defined(linux)
-#undef BN_DIV2W
-#define BN_DIV2W
+# ifndef BN_DIV2W
+#  define BN_DIV2W
+# endif
 #endif
 
 /* assuming long is 64bit - this is the DEC Alpha