diff --git a/lib/help_scout.rb b/lib/help_scout.rb
index 4a3c671..dae9831 100644
--- a/lib/help_scout.rb
+++ b/lib/help_scout.rb
@@ -60,7 +60,7 @@ def get_conversation(id)
   def get_conversations(mailbox_id, page = 1, modified_since = nil)
     options ={
       page: page,
-      modified_since: modified_since,
+      modifiedSince: modified_since,
     }
     get("mailboxes/#{mailbox_id}/conversations", options)
   end