diff --git a/eventlet/green/thread.py b/eventlet/green/thread.py
index 7d92ec6..9a6ac78 100644
--- a/eventlet/green/thread.py
+++ b/eventlet/green/thread.py
@@ -38,8 +38,8 @@ def interrupt_main():
     else:
         raise KeyboardInterrupt()
 
-__original_stack_size__ = __thread.stack_size
 if hasattr(__thread, 'stack_size'):
+    __original_stack_size__ = __thread.stack_size
     def stack_size(size=None):
         if size is None:
             return __original_stack_size__()