title: Why slack gives me invalid trigger error?
body: <p>Trying to send a dialog to slack channel via the <code>dialog.open</code> api, but always get the error:</p>
  
  <blockquote>
  <pre><code>trigger id 305394181715.285081825728.e91e087e5199b235c8678aeaaedf6450
  type of trigger id string 
  { 
      ok: false,   
      error: 'invalid_trigger',  
      warning: 'missing_charset',   
      response_metadata: { 
          warnings: [
              'missing_charset' 
          ] 
      } 
  }
  </code></pre>
  </blockquote>
  
  <p>I am using node.js, here is how my code looks like:</p>
  
  <pre><code>dialog_url = "https://slack.com/api/dialog.open"
  
  dialog = {
    "callback_id": "ryde-46e2b0",
    "title": "Request a Ride",
    "submit_label": "Request",
    "elements": [
      {
        "type": "text",
        "label": "Pickup Location",
        "name": "loc_origin"
      },
      {
        "type": "text",
        "label": "Dropoff Location",
        "name": "loc_destination"
      }
    ]
  }
  
  // handle the post triggered by slash command using node express
  app.post('/dialog', (req, res) =&gt; {
  
    console.log('trigger id', req.body.trigger_id)
    console.log('type of trigger id', typeof(req.body.trigger_id))
  
    // post dialog to dialog url
    axios.post(dialog_url, { 
      dialog, 
      trigger_id: req.body.trigger_id
    }, {
      headers: {
        'Content-type': 'application/json',
        'charset': 'UTF-8',
        'Authorization': process.env.token || process.env.bot_token
      }
    }).then(res =&gt; {
      console.log(res.data)
    }).catch(err =&gt; console.log(err))
  })
  </code></pre>
  
  <p>Is there someone experienced to point me to the right direction? Many thanks in advance.</p>