diff --git a/lib/XMLHttpRequest.js b/lib/XMLHttpRequest.js
index 949fdf9..94ba128 100644
--- a/lib/XMLHttpRequest.js
+++ b/lib/XMLHttpRequest.js
@@ -348,7 +348,7 @@ exports.XMLHttpRequest = function() {
     } else if (data) {
       headers["Content-Length"] = Buffer.isBuffer(data) ? data.length : Buffer.byteLength(data);
 
-      if (!headers["Content-Type"]) {
+      if (Object.keys(headers).filter(function(key) { return key.toLowerCase() === 'content-type'; }).length === 0) {
         headers["Content-Type"] = "text/plain;charset=UTF-8";
       }
     } else if (settings.method === "POST") {