diff --git a/federation/pdu/pdu.go b/federation/pdu/pdu.go
index cecee5b9..17db6995 100644
--- a/federation/pdu/pdu.go
+++ b/federation/pdu/pdu.go
@@ -123,6 +123,19 @@ func (pdu *PDU) ToClientEvent(roomVersion id.RoomVersion) (*event.Event, error)
 	return evt, nil
 }
 
+func (pdu *PDU) AddSignature(serverName string, keyID id.KeyID, signature string) {
+	if signature == "" {
+		return
+	}
+	if pdu.Signatures == nil {
+		pdu.Signatures = make(map[string]map[id.KeyID]string)
+	}
+	if _, ok := pdu.Signatures[serverName]; !ok {
+		pdu.Signatures[serverName] = make(map[id.KeyID]string)
+	}
+	pdu.Signatures[serverName][keyID] = signature
+}
+
 func marshalCanonical(data any) (jsontext.Value, error) {
 	marshaledBytes, err := json.Marshal(data)
 	if err != nil {
diff --git a/federation/pdu/signature.go b/federation/pdu/signature.go
index a7685cc6..04e7c5ef 100644
--- a/federation/pdu/signature.go
+++ b/federation/pdu/signature.go
@@ -28,13 +28,7 @@ func (pdu *PDU) Sign(roomVersion id.RoomVersion, serverName string, keyID id.Key
 		return fmt.Errorf("failed to marshal redacted PDU to sign: %w", err)
 	}
 	signature := ed25519.Sign(privateKey, rawJSON)
-	if pdu.Signatures == nil {
-		pdu.Signatures = make(map[string]map[id.KeyID]string)
-	}
-	if _, ok := pdu.Signatures[serverName]; !ok {
-		pdu.Signatures[serverName] = make(map[id.KeyID]string)
-	}
-	pdu.Signatures[serverName][keyID] = base64.RawStdEncoding.EncodeToString(signature)
+	pdu.AddSignature(serverName, keyID, base64.RawStdEncoding.EncodeToString(signature))
 	return nil
 }