diff --git a/substrate/primitives/runtime/src/transaction_validity.rs b/substrate/primitives/runtime/src/transaction_validity.rs
index 2191e59b9b..74709d9ae9 100644
--- a/substrate/primitives/runtime/src/transaction_validity.rs
+++ b/substrate/primitives/runtime/src/transaction_validity.rs
@@ -54,6 +54,13 @@ pub enum InvalidTransaction {
 	/// it will only be able to assume a bad signature and cannot express a more meaningful error.
 	BadProof,
 	/// The transaction birth block is ancient.
+	///
+	/// # Possible causes
+	///
+	/// For `FRAME`-based runtimes this would be caused by `current block number
+	/// - Era::birth block number > BlockHashCount`. (e.g. in Polkadot `BlockHashCount` = 2400, so a
+	/// transaction with birth block number 1337 would be valid up until block number 1337 + 2400,
+	/// after which point the transaction would be considered to have an ancient birth block.)
 	AncientBirthBlock,
 	/// The transaction would exhaust the resources of current block.
 	///