diff --git a/src/s3_lambda_es.js b/src/s3_lambda_es.js
index 4e14b05..65dc7b9 100644
--- a/src/s3_lambda_es.js
+++ b/src/s3_lambda_es.js
@@ -82,7 +82,8 @@ function postDocumentToES(doc, context) {
     req.body = doc;
     req.headers['presigned-expires'] = false;
     req.headers['Host'] = endpoint.host;
-
+    req.headers['Content-Type'] = 'application/json';
+    
     // Sign the request (Sigv4)
     var signer = new AWS.Signers.V4(req, 'es');
     signer.addAuthorization(creds, new Date());