diff --git a/Python/thread.c b/Python/thread.c
index acb2103..f5722a7 100644
--- a/Python/thread.c
+++ b/Python/thread.c
@@ -114,6 +114,7 @@
 #endif
 
 #ifdef HAVE_PTH
+#error GNU pth threads are now unsupported, and code will be removed in 3.3.
 #include "thread_pth.h"
 #undef _POSIX_THREADS
 #endif