diff --git a/types/heartbeat.go b/types/heartbeat.go
index 1bd2611f..4f0ad5ff 100644
--- a/types/heartbeat.go
+++ b/types/heartbeat.go
@@ -10,7 +10,7 @@ type HeartbeatTxnFields struct {
 	HbAddress Address `codec:"hbad"`
 
 	// HbProof is a signature using HeartbeatAddress's partkey, thereby showing it is online.
-	HbProof OneTimeSignature `codec:"hbprf"`
+	HbProof HeartbeatProof `codec:"hbprf"`
 
 	// HbSeed must be the block seed for the block before this transaction's
 	// firstValid. It is supplied in the transaction so that Proof can be
diff --git a/types/signature.go b/types/signature.go
index a1f0d824..bfa64570 100644
--- a/types/signature.go
+++ b/types/signature.go
@@ -84,6 +84,9 @@ func (lsig LogicSig) Blank() bool {
 type ed25519Signature [64]byte
 type ed25519PublicKey [32]byte
 
+// A HeartbeatProof is functionally equivalent to a OneTimeSignature (see below), but it has
+// been cleaned up for use as a transaction field in heartbeat transactions.
+//
 // A OneTimeSignature is a cryptographic signature that is produced a limited
 // number of times and provides forward integrity.
 //
@@ -92,26 +95,17 @@ type ed25519PublicKey [32]byte
 // identifier, the corresponding secret is deleted. This prevents the
 // secret-holder from signing a contradictory message in the future in the event
 // of a secret-key compromise.
-type OneTimeSignature struct {
-	// Unfortunately we forgot to mark this struct as omitempty at
-	// one point, and now it's hard to change if we want to preserve
-	// encodings.
-	_struct struct{} `codec:""`
+type HeartbeatProof struct {
+	_struct struct{} `codec:",omitempty,omitemptyarray"`
 
 	// Sig is a signature of msg under the key PK.
 	Sig ed25519Signature `codec:"s"`
 	PK  ed25519PublicKey `codec:"p"`
 
-	// Old-style signature that does not use proper domain separation.
-	// PKSigOld is unused; however, unfortunately we forgot to mark it
-	// `codec:omitempty` and so it appears (with zero value) in certs.
-	// This means we can't delete the field without breaking catchup.
-	PKSigOld ed25519Signature `codec:"ps"`
-
-	// Used to verify a new-style two-level ephemeral signature.
+	// PK2 is used to verify a two-level ephemeral signature.
+	PK2 ed25519PublicKey `codec:"p2"`
 	// PK1Sig is a signature of OneTimeSignatureSubkeyOffsetID(PK, Batch, Offset) under the key PK2.
-	// PK2Sig is a signature of OneTimeSignatureSubkeyBatchID(PK2, Batch) under the master key (OneTimeSignatureVerifier).
-	PK2    ed25519PublicKey `codec:"p2"`
 	PK1Sig ed25519Signature `codec:"p1s"`
+	// PK2Sig is a signature of OneTimeSignatureSubkeyBatchID(PK2, Batch) under the master key (OneTimeSignatureVerifier).
 	PK2Sig ed25519Signature `codec:"p2s"`
 }