make_new_cpu_set takes the number of cpus
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index bb13f47..d4ba9cf 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -4923,7 +4923,7 @@
             Py_INCREF(res); \
         } \
         else { \
-            res = make_new_cpu_set(&cpu_set_type, left->size); \
+            res = make_new_cpu_set(&cpu_set_type, left->ncpus); \
             if (!res) \
                 return NULL; \
         } \