diff --git a/src/base-apis.js b/src/base-apis.js
index 6491b1510..ce74ce8ed 100644
--- a/src/base-apis.js
+++ b/src/base-apis.js
@@ -1757,7 +1757,7 @@ MatrixBaseApis.prototype.downloadKeysForUsers = function(userIds, opts) {
         content.token = opts.token;
     }
     userIds.forEach((u) => {
-        content.device_keys[u] = {};
+        content.device_keys[u] = [];
     });
 
     return this._http.authedRequest(undefined, "POST", "/keys/query", undefined, content);