diff --git a/types/tx.go b/types/tx.go
index e8cde4c4..da3dc2cb 100644
--- a/types/tx.go
+++ b/types/tx.go
@@ -84,7 +84,7 @@ func (txs Txs) Proof(i int) TxProof {
 // TxProof represents a Merkle proof of the presence of a transaction in the Merkle tree.
 type TxProof struct {
 	RootHash cmn.HexBytes       `json:"root_hash"`
-	Data     Tx                 `json:"Data"`
+	Data     Tx                 `json:"data"`
 	Proof    merkle.SimpleProof `json:"proof"`
 }