diff --git a/crypto/opensslconf.h.in b/crypto/opensslconf.h.in
index 1b85ae598..5da591122 100644
--- a/crypto/opensslconf.h.in
+++ b/crypto/opensslconf.h.in
@@ -9,6 +9,7 @@
 #endif
 #endif
 
+#undef OPENSSL_UNISTD
 #define OPENSSL_UNISTD <unistd.h>
 
 #if defined(HEADER_IDEA_H) && !defined(IDEA_INT)