diff --git a/proposals/4140-delayed-events-futures.md b/proposals/4140-delayed-events-futures.md
index 409adcdc9..3aed98e7e 100644
--- a/proposals/4140-delayed-events-futures.md
+++ b/proposals/4140-delayed-events-futures.md
@@ -126,7 +126,7 @@ Content-Type: application/json
 }
 ```
 
-The `delay_id` is generated by the server. It SHOULD contain only valid non-surrogate Unicode codepoints except `:` and `NUL`, and MUST NOT exceed 255 bytes.
+The `delay_id` is an [opaque identifier](https://spec.matrix.org/v1.16/appendices/#opaque-identifiers) generated by the server.
 
 The server MAY round the delay up to a maximum of 30 seconds away from the request.