Fix instantiating validators with cached refs-to-bool schemas.

Closes: #1018

CHANGELOG.rst

@@ -1,3 +1,9 @@
+v4.17.3
+=======
+
+* Fix instantiating validators with cached refs to boolean schemas
+  rather than objects (#1018).
+
 v4.17.2
 =======
 

jsonschema/tests/test_validators.py

@@ -1848,6 +1848,21 @@ class TestDraft202012Validator(ValidatorTestMixin, TestCase):
     invalid = {"type": "integer"}, "foo"
 
 
+class TestLatestValidator(TestCase):
+    """
+    These really apply to multiple versions but are easiest to test on one.
+    """
+
+    def test_ref_resolvers_may_have_boolean_schemas_stored(self):
+        ref = "someCoolRef"
+        schema = {"$ref": ref}
+        resolver = validators.RefResolver("", {}, store={ref: False})
+        validator = validators._LATEST_VERSION(schema, resolver=resolver)
+
+        with self.assertRaises(exceptions.ValidationError):
+            validator.validate(None)
+
+
 class TestValidatorFor(TestCase):
     def test_draft_3(self):
         schema = {"$schema": "http://json-schema.org/draft-03/schema"}

jsonschema/validators.py

@@ -4,7 +4,7 @@
 from __future__ import annotations
 
 from collections import deque
-from collections.abc import Sequence
+from collections.abc import Mapping, Sequence
 from functools import lru_cache
 from operator import methodcaller
 from urllib.parse import unquote, urldefrag, urljoin, urlsplit
@@ -745,7 +745,8 @@ def __init__(
         self.store.update(store)
         self.store.update(
             (schema["$id"], schema)
-            for schema in store.values() if "$id" in schema
+            for schema in store.values()
+            if isinstance(schema, Mapping) and "$id" in schema
         )
         self.store[base_uri] = referrer