diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -6653,9 +6653,12 @@ posix_confstr(PyObject *self, PyObject *
         }
         else {
             if ((unsigned int)len >= sizeof(buffer)) {
-                result = PyUnicode_FromStringAndSize(NULL, len-1);
-                if (result != NULL)
-                    confstr(name, _PyUnicode_AsString(result), len);
+                char *buf = PyMem_Malloc(len);
+                if (buf == NULL)
+                    return PyErr_NoMemory();
+                confstr(name, buf, len);
+                result = PyUnicode_FromStringAndSize(buf, len-1);
+                PyMem_Free(buf);
             }
             else
                 result = PyUnicode_FromStringAndSize(buffer, len-1);