Discord Python Reaction Role

I want that my bot add the role "User" to a user who react with a heart. But it doesn't work.
My code:

@client.event
async def on_reaction_add(reaction, user):
    if reaction.emoji == "❤️":
        user = discord.utils.get(user.server.roles, name="User")
        await user.add_roles(user)

I hope you can help me :)

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