From 3e26e42c905852394fa136f1cc564dac98b56166 Mon Sep 17 00:00:00 2001
From: Zackery Spytz <zspytz@gmail.com>
Date: Mon, 20 Aug 2018 21:11:40 -0600
Subject: [PATCH] bpo-34400: Fix more undefined behavior in parsetok.c
 (GH-8833)

---
 Parser/parsetok.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/Parser/parsetok.c b/Parser/parsetok.c
index b9c9fe8fa8c..a1580e6751e 100644
--- a/Parser/parsetok.c
+++ b/Parser/parsetok.c
@@ -252,11 +252,13 @@ parsetok(struct tok_state *tok, grammar *g, int start, perrdetail *err_ret,
             }
         }
 #endif
-        if (a >= tok->line_start)
+        if (a != NULL && a >= tok->line_start) {
             col_offset = Py_SAFE_DOWNCAST(a - tok->line_start,
                                           intptr_t, int);
-        else
+        }
+        else {
             col_offset = -1;
+        }
 
         if ((err_ret->error =
              PyParser_AddToken(ps, (int)type, str,