diff --git a/changelog.d/5258.bugfix b/changelog.d/5258.bugfix
new file mode 100644
index 0000000000..fb5d44aedb
--- /dev/null
+++ b/changelog.d/5258.bugfix
@@ -0,0 +1 @@
+Fix error when downloading thumbnail with missing width/height parameter.
diff --git a/synapse/rest/media/v1/thumbnail_resource.py b/synapse/rest/media/v1/thumbnail_resource.py
index 5305e9175f..35a750923b 100644
--- a/synapse/rest/media/v1/thumbnail_resource.py
+++ b/synapse/rest/media/v1/thumbnail_resource.py
@@ -56,8 +56,8 @@ class ThumbnailResource(Resource):
     def _async_render_GET(self, request):
         set_cors_headers(request)
         server_name, media_id, _ = parse_media_id(request)
-        width = parse_integer(request, "width")
-        height = parse_integer(request, "height")
+        width = parse_integer(request, "width", required=True)
+        height = parse_integer(request, "height", required=True)
         method = parse_string(request, "method", "scale")
         m_type = parse_string(request, "type", "image/png")