diff --git a/firmware/ethereum.c b/firmware/ethereum.c
index 87b7f529da..a5cf7a32a7 100644
--- a/firmware/ethereum.c
+++ b/firmware/ethereum.c
@@ -568,7 +568,8 @@ void ethereum_signing_init(EthereumSignTx *msg, const HDNode *node)
 		rlp_length += rlp_calculate_length(1, tx_type);
 	}
 	if (chain_id) {
-		rlp_length += rlp_calculate_length(1, chain_id);
+		int length = chain_id < 0x100 ? 1: chain_id < 0x10000 ? 2: chain_id < 0x1000000 ? 3 : 4;
+		rlp_length += rlp_calculate_length(length, chain_id);
 		rlp_length += rlp_calculate_length(0, 0);
 		rlp_length += rlp_calculate_length(0, 0);
 	}