diff --git a/synapse/handlers/appservice.py b/synapse/handlers/appservice.py
index 321fb24d71..48a328b692 100644
--- a/synapse/handlers/appservice.py
+++ b/synapse/handlers/appservice.py
@@ -19,6 +19,7 @@
 #
 #
 import logging
+from sys import exc_info
 from typing import (
     TYPE_CHECKING,
     Any,
@@ -94,7 +95,7 @@ class ApplicationServiceUrlPreviewResult(ParseModel):
         def check_str(key):
             if self.result:
                 v = self.result.get(key)
-                if v is not None and isinstance(v, str):
+                if v is not None and not isinstance(v, str):
                     raise ValueError(f"'{key}' must be a string if defined")
 
         # Non-exhaustive list of keys that should be strings based on https://ogp.me/.
@@ -777,7 +778,16 @@ class ApplicationServicesHandler:
                 exclusive_appservice, url, user_id
             )
             url_previews_counter.labels(**{SERVER_NAME_LABEL: self.server_name}).inc(1)
-            return ApplicationServiceUrlPreviewResult(exclusive=True, result=result)
+            try:
+                return ApplicationServiceUrlPreviewResult(exclusive=True, result=result)
+            except Exception:
+                logger.warning(
+                    "exclusive appservice preview from %s was not valid",
+                    exclusive_appservice.id,
+                    exc_info=True,
+                )
+                return ApplicationServiceUrlPreviewResult(exclusive=True, result=None)
+
         for appservice in self._get_services_for_preview_url(url=url):
             result = await self.appservice_api.query_preview_url(
                 appservice, url, user_id
@@ -786,10 +796,21 @@ class ApplicationServicesHandler:
                 url_previews_counter.labels(
                     **{SERVER_NAME_LABEL: self.server_name}
                 ).inc(1)
-                return ApplicationServiceUrlPreviewResult(
-                    exclusive=False,
-                    result=result,
-                )
+
+                try:
+                    return ApplicationServiceUrlPreviewResult(
+                        exclusive=False,
+                        result=result,
+                    )
+                except Exception:
+                    logger.warning(
+                        "appservice preview from %s was not valid",
+                        appservice.id,
+                        exc_info=True,
+                    )
+                    return ApplicationServiceUrlPreviewResult(
+                        exclusive=True, result=None
+                    )
         return ApplicationServiceUrlPreviewResult(
             exclusive=False,
             result=None,
diff --git a/tests/handlers/test_appservice.py b/tests/handlers/test_appservice.py
index 4dafe4d2d4..6a702eb67a 100644
--- a/tests/handlers/test_appservice.py
+++ b/tests/handlers/test_appservice.py
@@ -258,6 +258,18 @@ class AppServiceHandlerTestCase(unittest.TestCase):
         self.assertEqual(result.result, return_value)
         self.assertTrue(result.exclusive)
 
+    async def test_query_url_preview_rejects_invalid(self) -> None:
+        self.mock_store.get_app_services.return_value = [
+            self._mkservice_url_preview(True, True)
+        ]
+        self.mock_as_api.query_preview_url = AsyncMock(return_value={"og:title": True})
+        result = await self.handler.query_preview_url(
+            "https://matrix.org", UserID(localpart="a", domain="b")
+        )
+        assert result is not None
+        self.assertEqual(result.result, None)
+        self.assertTrue(result.exclusive)
+
     async def test_query_url_preview_multiple_services(self) -> None:
         return_value = {"og:title": "foobar"}
         url_service = self._mkservice_url_preview()