diff --git a/src/crypto/algorithms/megolm.ts b/src/crypto/algorithms/megolm.ts
index af80a9489..e111703a8 100644
--- a/src/crypto/algorithms/megolm.ts
+++ b/src/crypto/algorithms/megolm.ts
@@ -1539,7 +1539,7 @@ class MegolmDecryption extends DecryptionAlgorithm {
             await olmlib.encryptMessageForDevice(
                 encryptedContent.ciphertext,
                 this.userId,
-                this.deviceId,
+                undefined,
                 this.olmDevice,
                 sender,
                 device,
@@ -1619,7 +1619,7 @@ class MegolmDecryption extends DecryptionAlgorithm {
             return this.olmlib.encryptMessageForDevice(
                 encryptedContent.ciphertext,
                 this.userId,
-                this.deviceId,
+                undefined,
                 this.olmDevice,
                 userId,
                 deviceInfo,
@@ -1782,7 +1782,7 @@ class MegolmDecryption extends DecryptionAlgorithm {
                         olmlib.encryptMessageForDevice(
                             encryptedContent.ciphertext,
                             this.userId,
-                            this.deviceId,
+                            undefined,
                             this.olmDevice,
                             userId,
                             deviceInfo,
diff --git a/src/crypto/olmlib.ts b/src/crypto/olmlib.ts
index f7ed067ee..7a5e3a26c 100644
--- a/src/crypto/olmlib.ts
+++ b/src/crypto/olmlib.ts
@@ -97,6 +97,7 @@ export async function encryptMessageForDevice(
 
     const payload = {
         sender: ourUserId,
+        // TODO this appears to no longer be used whatsoever
         sender_device: ourDeviceId,
 
         // Include the Ed25519 key so that the recipient knows what