diff --git a/changelog.d/11988.bugfix b/changelog.d/11988.bugfix
new file mode 100644
index 0000000000000000000000000000000000000000..ced10d0c8156f7f0da968f66ac9e01371a00a390
--- /dev/null
+++ b/changelog.d/11988.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug where `M_WRONG_ROOM_KEYS_VERSION` errors would not include the specced `current_version` field.
diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index 85302163dad945ae6016645fb6f663ae4df5b65c..e92db29f6dc6a65a0e05ca9fc62e409773b34b6c 100644
--- a/synapse/api/errors.py
+++ b/synapse/api/errors.py
@@ -406,6 +406,9 @@ class RoomKeysVersionError(SynapseError):
         super().__init__(403, "Wrong room_keys version", Codes.WRONG_ROOM_KEYS_VERSION)
         self.current_version = current_version
 
+    def error_dict(self) -> "JsonDict":
+        return cs_error(self.msg, self.errcode, current_version=self.current_version)
+
 
 class UnsupportedRoomVersionError(SynapseError):
     """The client's request to create a room used a room version that the server does