title: How to append/concatenate strings in mule Transform message?
body: <p>I am trying to concatenate strings in mule Transform message like below, but I am getting the below exception at run time. Could anyone please kindly help me on this? I am new to mule as well.</p>
  
  <pre><code>%dw 1.0
  %output application/json
  ---
  {
      references : "" when payload[0].person_id==null otherwise "person/"+payload[0].person_id,
  }
  </code></pre>
  
  <p>Exception :: </p>
  
  <pre><code>Root Exception stack trace:
  com.mulesoft.weave.mule.exception.WeaveExecutionException: Exception while executing: 
      references : "" when payload[0].person_id==null otherwise "person/"+payload[0].person_id,
                                                                ^
  Type mismatch for '+' operator
       found :string, :number
  </code></pre>