diff --git a/spawn-pipe.c b/spawn-pipe.c
index b3f9e0c..965fb3a 100644
--- a/spawn-pipe.c
+++ b/spawn-pipe.c
@@ -48,6 +48,13 @@
 
 #endif
 
+/* environ is the exported symbol referencing the internal
+   __cygwin_environ variable on cygwin64:
+   <https://cygwin.com/ml/cygwin/2013-06/msg00228.html>.  */
+#if defined __CYGWIN__ && defined __x86_64__
+extern DLL_VARIABLE char **environ;
+#endif
+
 /* The results of open() in this file are not used with fchdir,
    therefore save some unnecessary work in fchdir.c.  */
 #undef open