diff --git a/src/crypto/CrossSigning.js b/src/crypto/CrossSigning.js
index 9e877dcfc0d..c0633e958af 100644
--- a/src/crypto/CrossSigning.js
+++ b/src/crypto/CrossSigning.js
@@ -644,6 +644,11 @@ export function createCryptoStoreCacheCallbacks(store) {
             });
         },
         storeCrossSigningKeyCache: function(type, key) {
+            if (!(key instanceof Uint8Array)) {
+                throw new Error(
+                    `storeCrossSigningKeyCache expects Uint8Array, got ${key}`,
+                );
+            }
             return store.doTxn(
                 'readwrite',
                 [IndexedDBCryptoStore.STORE_ACCOUNT],
diff --git a/src/crypto/index.js b/src/crypto/index.js
index 7ee23161616..96ddc218c02 100644
--- a/src/crypto/index.js
+++ b/src/crypto/index.js
@@ -822,6 +822,9 @@ Crypto.prototype.getSessionBackupPrivateKey = async function() {
  * @returns {Promise} so you can catch failures
  */
 Crypto.prototype.storeSessionBackupPrivateKey = async function(key) {
+    if (!(key instanceof Uint8Array)) {
+        throw new Error(`storeSessionBackupPrivateKey expects Uint8Array, got ${key}`);
+    }
     return this._cryptoStore.doTxn(
         'readwrite',
         [IndexedDBCryptoStore.STORE_ACCOUNT],