diff --git a/conformance/server.rb b/conformance/server.rb
index 256a1d4..3205f5b 100644
--- a/conformance/server.rb
+++ b/conformance/server.rb
@@ -942,7 +942,9 @@ module Conformance
             ).to_h,
           ]
         else
-          []
+          # Per SEP-2164, an unknown URI answers with -32602 carrying the URI in `error.data`;
+          # an empty `contents` array is forbidden.
+          raise MCP::Server::ResourceNotFoundError.new(uri, params)
         end
       end
     end