diff --git a/changelog.d/3391.bugfix b/changelog.d/3391.bugfix
new file mode 100644
index 000000000..88eeb50df
--- /dev/null
+++ b/changelog.d/3391.bugfix
@@ -0,0 +1 @@
+Default inviter_display_name to mxid for email invites
\ No newline at end of file
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index a832d9180..0d4a3f467 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -708,6 +708,10 @@ class RoomMemberHandler(object):
             inviter_display_name = member_event.content.get("displayname", "")
             inviter_avatar_url = member_event.content.get("avatar_url", "")
 
+        # if user has no display name, default to their MXID
+        if not inviter_display_name:
+            inviter_display_name = user.to_string()
+
         canonical_room_alias = ""
         canonical_alias_event = room_state.get((EventTypes.CanonicalAlias, ""))
         if canonical_alias_event: