diff --git a/Python/thread.c b/Python/thread.c
index 5b8ec10..acb2103 100644
--- a/Python/thread.c
+++ b/Python/thread.c
@@ -109,6 +109,7 @@
 #endif
 
 #ifdef SUN_LWP
+#error SunOS lightweight processes are now unsupported, and code will be removed in 3.3.
 #include "thread_lwp.h"
 #endif