https://hg-edge.mozilla.org/mozilla-central/rev/d4b3eb4f76e8

Index: third_party/python/jsonschema/jsonschema/validators.py
--- third_party/python/jsonschema/jsonschema/validators.py.orig
+++ third_party/python/jsonschema/jsonschema/validators.py
@@ -875,8 +875,11 @@ class RefResolver:
             return None
         uri, fragment = urldefrag(url)
         for subschema in subschemas:
+            id = subschema["$id"]
+            if not isinstance(id, str):
+                continue
             target_uri = self._urljoin_cache(
-                self.resolution_scope, subschema["$id"],
+                self.resolution_scope, id,
             )
             if target_uri.rstrip("/") == uri.rstrip("/"):
                 if fragment:
