diff --git a/crates/ruma-serde/src/base64.rs b/crates/ruma-serde/src/base64.rs
index 516b0100..a2ac16bf 100644
--- a/crates/ruma-serde/src/base64.rs
+++ b/crates/ruma-serde/src/base64.rs
@@ -9,6 +9,7 @@ pub struct Base64<B = Vec<u8>> {
     bytes: B,
 }
 
+// See https://github.com/matrix-org/matrix-doc/issues/3211
 const BASE64_CONFIG: base64::Config = base64::STANDARD_NO_PAD.decode_allow_trailing_bits(true);
 
 impl<B: AsRef<[u8]>> Base64<B> {