title: Mule:Retrieving object store in an MEL in Mule 3.5
body: <p>Having a requirement to test a object store whether it contains a key or not in a choice router</p>
  
  <pre><code>&lt;objectstore:config name="storeDownload" doc:name="ObjectStore" persistent="false" partition="test"/&gt;
  
        &lt;choice&gt;
           &lt;when  expression="#[app.registry.storeDownload.contains('#[flowVars.startKey]').equals('false')]"&gt;
  </code></pre>
  
  <p>Getting an error 
  1. Expression Evaluator "registry" with expression "ON" returned null but a value was required. (org.mule.api.expression.ExpressionRuntimeException)
    org.mule.expression.RegistryExpressionEvaluator:101 (<a href="http://www.mulesoft.org/docs/site/current3/apidocs/org/mule/api/expression/ExpressionRuntimeException.html" rel="nofollow">http://www.mulesoft.org/docs/site/current3/apidocs/org/mule/api/expression/ExpressionRuntimeException.html</a>)
  2. Failed to invoke store. Message payload is of type: byte[] (org.mule.api.MessagingException)</p>