diff --git a/src/@types/requests.ts b/src/@types/requests.ts
index 0c79534e8..eca0132fb 100644
--- a/src/@types/requests.ts
+++ b/src/@types/requests.ts
@@ -181,6 +181,12 @@ export interface IBindThreePidBody {
     sid: string;
 }
 
+export interface IAddThreePidBody {
+    client_secret: string;
+    id_server: string;
+    sid: string;
+}
+
 export interface IRelationsRequestOpts {
     from?: string;
     to?: string;
diff --git a/src/client.ts b/src/client.ts
index 16da17917..b8568a0a3 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -133,6 +133,7 @@ import {
     IFilterResponse,
     ITagsResponse,
     IStatusResponse,
+    IAddThreePidBody,
 } from "./@types/requests";
 import {
     EventType,
@@ -8519,7 +8520,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      * @returns Promise which resolves: on success
      * @returns Rejects: with an error response.
      */
-    public addThreePid(creds: IBindThreePidBody, bind: boolean): Promise<{ submit_url?: string }> {
+    public addThreePid(creds: IAddThreePidBody, bind: boolean): Promise<{ submit_url?: string }> {
         const path = "/account/3pid";
         const data = {
             threePidCreds: creds,
diff --git a/src/crypto/index.ts b/src/crypto/index.ts
index beeefa546..ab588afce 100644
--- a/src/crypto/index.ts
+++ b/src/crypto/index.ts
@@ -1242,20 +1242,22 @@ export class Crypto extends TypedEventEmitter<CryptoEvent, CryptoEventHandlerMap
      * @returns the key, if any, or null
      */
     public async getSessionBackupPrivateKey(): Promise<Uint8Array | null> {
-        let key = await new Promise<Uint8Array | IEncryptedPayload | null>((resolve) => {
+        const encodedKey = await new Promise<Uint8Array | IEncryptedPayload | string | null>((resolve) => {
             this.cryptoStore.doTxn("readonly", [IndexedDBCryptoStore.STORE_ACCOUNT], (txn) => {
                 this.cryptoStore.getSecretStorePrivateKey(txn, resolve, "m.megolm_backup.v1");
             });
         });
 
+        let key: Uint8Array | null = null;
+
         // make sure we have a Uint8Array, rather than a string
-        if (key && typeof key === "string") {
-            key = new Uint8Array(olmlib.decodeBase64(fixBackupKey(key) || key));
+        if (typeof encodedKey === "string") {
+            key = new Uint8Array(olmlib.decodeBase64(fixBackupKey(encodedKey) || encodedKey));
             await this.storeSessionBackupPrivateKey(key);
         }
-        if (key && typeof key === "object" && "ciphertext" in key) {
+        if (encodedKey && typeof encodedKey === "object" && "ciphertext" in encodedKey) {
             const pickleKey = Buffer.from(this.olmDevice.pickleKey);
-            const decrypted = await decryptAES(key, pickleKey, "m.megolm_backup.v1");
+            const decrypted = await decryptAES(encodedKey, pickleKey, "m.megolm_backup.v1");
             key = olmlib.decodeBase64(decrypted);
         }
         return key;