diff --git a/ethcore/types/src/transaction/transaction.rs b/ethcore/types/src/transaction/transaction.rs
index f9b49c719cd..170a74f3e0c 100644
--- a/ethcore/types/src/transaction/transaction.rs
+++ b/ethcore/types/src/transaction/transaction.rs
@@ -239,6 +239,8 @@ impl Transaction {
 	}
 
 	/// Legacy EIP-86 compatible empty signature.
+	/// This method is used in json tests as well as
+	/// signature verification tests.
 	#[cfg(any(test, feature = "test-helpers"))]
 	pub fn null_sign(self, chain_id: u64) -> SignedTransaction {
 		SignedTransaction {