diff --git a/changelog.d/5638.bugfix b/changelog.d/5638.bugfix
new file mode 100644
index 0000000000..66781ad9e6
--- /dev/null
+++ b/changelog.d/5638.bugfix
@@ -0,0 +1 @@
+Fix requests to the `/store_invite` endpoint of identity servers being sent in the wrong format.
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 679daaa074..e0196ef83e 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -29,7 +29,7 @@ from twisted.internet import defer
 import synapse.server
 import synapse.types
 from synapse.api.constants import EventTypes, Membership
-from synapse.api.errors import AuthError, Codes, SynapseError
+from synapse.api.errors import AuthError, Codes, HttpResponseException, SynapseError
 from synapse.types import RoomID, UserID
 from synapse.util.async_helpers import Linearizer
 from synapse.util.distributor import user_joined_room, user_left_room
@@ -872,9 +872,23 @@ class RoomMemberHandler(object):
             "sender_avatar_url": inviter_avatar_url,
         }
 
-        data = yield self.simple_http_client.post_urlencoded_get_json(
-            is_url, invite_config
-        )
+        try:
+            data = yield self.simple_http_client.post_json_get_json(
+                is_url, invite_config
+            )
+        except HttpResponseException as e:
+            # Some identity servers may only support application/x-www-form-urlencoded
+            # types. This is especially true with old instances of Sydent, see
+            # https://github.com/matrix-org/sydent/pull/170
+            logger.info(
+                "Failed to POST %s with JSON, falling back to urlencoded form: %s",
+                is_url,
+                e,
+            )
+            data = yield self.simple_http_client.post_urlencoded_get_json(
+                is_url, invite_config
+            )
+
         # TODO: Check for success
         token = data["token"]
         public_keys = data.get("public_keys", [])