From 850f4e72cd65cf596accf76287e7e1976b396a07 Mon Sep 17 00:00:00 2001
From: Patrick Kidger <33688385+patrick-kidger@users.noreply.github.com>
Date: Thu, 2 Nov 2023 20:01:54 -0700
Subject: [PATCH] Handle beartype doing isinstance(None, hint)

---
 jaxtyping/_pytree_type.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/jaxtyping/_pytree_type.py b/jaxtyping/_pytree_type.py
index 29ccb77..df89997 100644
--- a/jaxtyping/_pytree_type.py
+++ b/jaxtyping/_pytree_type.py
@@ -54,6 +54,10 @@ class _MetaPyTree(type):
     def __instancecheck__(cls, obj):
         if not hasattr(cls, "leaftype"):
             return True  # Just `isinstance(x, PyTree)`
+        # Handle beartype doing `isinstance(None, hint)` to check if
+        # is `instance`able.
+        if obj is None:
+            return True
 
         single_memo, variadic_memo, pytree_memo, arg_memo = get_shape_memo()
         single_memo_bak = single_memo.copy()