title: Validation Exception using DynamoDB and Amazon API Gateway
body: <p>hi I keep getting a validation exception error: </p>
  
  <p>"__type": "com.amazon.coral.validate#ValidationException",
    "message": "1 validation error detected: Value null at 'key' failed to satisfy constraint: Member must not be null"</p>
  
  <p>I have also attached what my application/json mapping template looks like. </p>
  
  <pre><code>{ 
  "TableName": "Comments",
  "Items": {
      "commentid": {
          "S": "$input.path('$.commentid')"
          },
      "pageid": {
          "S": "$input.path('$.pageid')"
          },
      "userName": {
          "S": "$input.path('$.userName')"
      },
      "message": {
          "S": "$input.path('$.message')"
      }
  }
  </code></pre>
  
  <p>}</p>
  
  <p>Thank You </p>