diff --git a/crates/matrix-sdk-crypto/src/machine/mod.rs b/crates/matrix-sdk-crypto/src/machine/mod.rs
index eb334ab7438..3624f4aa2cb 100644
--- a/crates/matrix-sdk-crypto/src/machine/mod.rs
+++ b/crates/matrix-sdk-crypto/src/machine/mod.rs
@@ -2564,6 +2564,13 @@ pub struct EncryptionSyncChanges<'a> {
     pub next_batch_token: Option<String>,
 }
 
+/// Convert a [`MegolmError`] into an [`UnableToDecryptInfo`] or a
+/// [`CryptoStoreError`].
+///
+/// Most `MegolmError` codes are converted into a suitable
+/// `UnableToDecryptInfo`. The exception is [`MegolmError::Store`], which
+/// represents a problem with our datastore rather than with the message itself,
+/// and is therefore returned as a `CryptoStoreError`.
 fn megolm_error_to_utd_info(
     raw_event: &Raw<EncryptedEvent>,
     error: MegolmError,