diff --git a/src/crypto/algorithms/megolm.js b/src/crypto/algorithms/megolm.js
index c9792b38e..b4a78334a 100644
--- a/src/crypto/algorithms/megolm.js
+++ b/src/crypto/algorithms/megolm.js
@@ -433,7 +433,15 @@ MegolmEncryption.prototype.reshareKeyWithDevice = async function(
     }
 
     // The chain index of the key we previously sent this device
+    if (obSessionInfo.sharedWithDevices[userId] === undefined) {
+        logger.debug("Session ID " + sessionId + " never shared with user " + userId);
+        return;
+    }
     const sentChainIndex = obSessionInfo.sharedWithDevices[userId][device.deviceId];
+    if (sentChainIndex === undefined) {
+        logger.debug("Session ID " + sessionId + " never shared with device " + userId + ":" + device.deviceId);
+        return;
+    }
 
     // get the key from the inbound session: the outbound one will already
     // have been ratcheted to the next chain index.
diff --git a/src/crypto/index.js b/src/crypto/index.js
index 54c6bb16a..7c0ac003a 100644
--- a/src/crypto/index.js
+++ b/src/crypto/index.js
@@ -1196,7 +1196,7 @@ Crypto.prototype._onToDeviceBadEncrypted = async function(event) {
     // We send this first such that, as long as the toDevice messages arrive in the
     // same order we sent them, the other end will get this first, set up the new session,
     // then get the keyshare request and send the key over this new session (because it
-    // it the session it has most recently received a message on).
+    // is the session it has most recently received a message on).
     const encryptedContent = {
         algorithm: olmlib.OLM_ALGORITHM,
         sender_key: this._olmDevice.deviceCurve25519Key,