diff --git a/synapse/rest/media/v1/preview_url_resource.py b/synapse/rest/media/v1/preview_url_resource.py
index c5ba83ddfd..6f896ffb53 100644
--- a/synapse/rest/media/v1/preview_url_resource.py
+++ b/synapse/rest/media/v1/preview_url_resource.py
@@ -137,7 +137,7 @@ class PreviewUrlResource(Resource):
         cache_result = yield self.store.get_url_cache(url, ts)
         if (
             cache_result and
-            cache_result["download_ts"] + cache_result["expires"] > ts and
+            cache_result["expires_ts"] > ts and
             cache_result["response_code"] / 100 == 2
         ):
             respond_with_json_bytes(
@@ -246,7 +246,7 @@ class PreviewUrlResource(Resource):
             url,
             media_info["response_code"],
             media_info["etag"],
-            media_info["expires"],
+            media_info["expires"] + media_info["created_ts"],
             json.dumps(og),
             media_info["filesystem_id"],
             media_info["created_ts"],
diff --git a/synapse/storage/media_repository.py b/synapse/storage/media_repository.py
index 5cca14ccb2..b8a0dd0762 100644
--- a/synapse/storage/media_repository.py
+++ b/synapse/storage/media_repository.py
@@ -62,7 +62,7 @@ class MediaRepositoryStore(SQLBaseStore):
         def get_url_cache_txn(txn):
             # get the most recently cached result (relative to the given ts)
             sql = (
-                "SELECT response_code, etag, expires, og, media_id, download_ts"
+                "SELECT response_code, etag, expires_ts, og, media_id, download_ts"
                 " FROM local_media_repository_url_cache"
                 " WHERE url = ? AND download_ts <= ?"
                 " ORDER BY download_ts DESC LIMIT 1"
@@ -74,7 +74,7 @@ class MediaRepositoryStore(SQLBaseStore):
                 # ...or if we've requested a timestamp older than the oldest
                 # copy in the cache, return the oldest copy (if any)
                 sql = (
-                    "SELECT response_code, etag, expires, og, media_id, download_ts"
+                    "SELECT response_code, etag, expires_ts, og, media_id, download_ts"
                     " FROM local_media_repository_url_cache"
                     " WHERE url = ? AND download_ts > ?"
                     " ORDER BY download_ts ASC LIMIT 1"
@@ -86,14 +86,14 @@ class MediaRepositoryStore(SQLBaseStore):
                 return None
 
             return dict(zip((
-                'response_code', 'etag', 'expires', 'og', 'media_id', 'download_ts'
+                'response_code', 'etag', 'expires_ts', 'og', 'media_id', 'download_ts'
             ), row))
 
         return self.runInteraction(
             "get_url_cache", get_url_cache_txn
         )
 
-    def store_url_cache(self, url, response_code, etag, expires, og, media_id,
+    def store_url_cache(self, url, response_code, etag, expires_ts, og, media_id,
                         download_ts):
         return self._simple_insert(
             "local_media_repository_url_cache",
@@ -101,7 +101,7 @@ class MediaRepositoryStore(SQLBaseStore):
                 "url": url,
                 "response_code": response_code,
                 "etag": etag,
-                "expires": expires,
+                "expires_ts": expires_ts,
                 "og": og,
                 "media_id": media_id,
                 "download_ts": download_ts,
@@ -242,8 +242,8 @@ class MediaRepositoryStore(SQLBaseStore):
     def get_expired_url_cache(self, now_ts):
         sql = (
             "SELECT media_id FROM local_media_repository_url_cache"
-            " WHERE download_ts + expires < ?"
-            " ORDER BY download_ts + expires ASC"
+            " WHERE expires_ts < ?"
+            " ORDER BY expires_ts ASC"
             " LIMIT 100"
         )
 
diff --git a/synapse/storage/schema/delta/44/expire_url_cache.sql b/synapse/storage/schema/delta/44/expire_url_cache.sql
index 997e790b6d..9475d53e84 100644
--- a/synapse/storage/schema/delta/44/expire_url_cache.sql
+++ b/synapse/storage/schema/delta/44/expire_url_cache.sql
@@ -14,4 +14,23 @@
  */
 
 CREATE INDEX local_media_repository_url_idx ON local_media_repository(created_ts) WHERE url_cache IS NOT NULL;
-CREATE INDEX local_media_repository_url_cache_expires_idx ON local_media_repository_url_cache((download_ts + expires));
+
+-- we need to change `expires` to `expires_ts` so that we can index on it. SQLite doesn't support
+-- indices on expressions until 3.9.
+CREATE TABLE local_media_repository_url_cache_new(
+    url TEXT,
+    response_code INTEGER,
+    etag TEXT,
+    expires_ts BIGINT,
+    og TEXT,
+    media_id TEXT,
+    download_ts BIGINT
+);
+
+INSERT INTO local_media_repository_url_cache_new
+    SELECT url, response_code, etag, expires + download_ts, og, media_id, download_ts FROM local_media_repository_url_cache;
+
+DROP TABLE local_media_repository_url_cache;
+ALTER TABLE local_media_repository_url_cache_new RENAME TO local_media_repository_url_cache;
+
+CREATE INDEX local_media_repository_url_cache_expires_idx ON local_media_repository_url_cache(expires_ts);