Simple HK Invoke question

When playing music on my Invoke, I'd sometimes like to stop playing the music after a period of time, say 30 minutes. When I do this on Alexa, I say "Alexa, set a sleep timer for 30 minutes" and the music stops after 30 minutes. When I do this on my Invoke speaker with Cortana, it sets a timer that rings after 30 minutes. What am I doing wrong? What is the correct syntax?

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