diff --git a/api/client-server/key_backup.yaml b/api/client-server/key_backup.yaml
index 387621e6..0f7ff25b 100644
--- a/api/client-server/key_backup.yaml
+++ b/api/client-server/key_backup.yaml
@@ -219,13 +219,25 @@ paths:
           schema:
             type: object
             properties: {}
+        400:
+          description: |-
+            A parameter was incorrect.  For example, the ``algorithm`` does not
+            match the current backup algorithm, or the ``version`` in the body
+            does not match the ``version`` in the path.
+          examples:
+            application/json: {
+              "errcode": "M_INVALID_PARAM",
+              "error": "Algorithm does not match"
+            }
+          schema:
+            "$ref": "definitions/errors/error.yaml"
         404:
           description: The backup specified does not exist.
           examples:
             application/json: {
-                "errcode": "M_NOT_FOUND",
-                "error": "Unknown backup version"
-              }
+              "errcode": "M_NOT_FOUND",
+              "error": "Unknown backup version"
+            }
           schema:
             "$ref": "definitions/errors/error.yaml"
       tags:
@@ -260,9 +272,9 @@ paths:
           description: The backup specified does not exist.
           examples:
             application/json: {
-                "errcode": "M_NOT_FOUND",
-                "error": "Unknown backup version"
-              }
+              "errcode": "M_NOT_FOUND",
+              "error": "Unknown backup version"
+            }
           schema:
             "$ref": "definitions/errors/error.yaml"
       tags:
diff --git a/specification/modules/end_to_end_encryption.rst b/specification/modules/end_to_end_encryption.rst
index d6c87bc3..8f26477a 100644
--- a/specification/modules/end_to_end_encryption.rst
+++ b/specification/modules/end_to_end_encryption.rst
@@ -846,9 +846,8 @@ following format:
    ========== =========== ======================================================
    public_key string      Required. The curve25519 public key used to encrypt
                           the backups, encoded in unpadded base64.
-   signatures {string:    Optional. Signatures of the ``auth_data``, as Signed
-              {string:    JSON
-              string}}
+   signatures Signatures  Optional. Signatures of the ``auth_data``, as Signed
+                          JSON
    ========== =========== ======================================================
 
 The ``session_data`` field in the backups is constructed as follows:
@@ -870,8 +869,9 @@ The ``session_data`` field in the backups is constructed as follows:
                                                `m.forwarded_room_key`_ events.
       sender_key                      string   Required. Unpadded base64-encoded
                                                device curve25519 key.
-      sender_claimed_keys             {string: Required. Object containing the
-                                      string}  identity key for the sending device.
+      sender_claimed_keys             {string: Required. A map from algorithm name
+                                      string}  (``ed25519``) to the identity key
+                                               for the sending device.
       session_key                     string   Required. Unpadded base64-encoded
                                                session key in `session-sharing format
                                                <https://gitlab.matrix.org/matrix-org/olm/blob/master/docs/megolm.md#session-sharing-format>`_.