diff --git a/spec/observables/dom/ajax-spec.ts b/spec/observables/dom/ajax-spec.ts
index 2849c5b5a4..9970376ca3 100644
--- a/spec/observables/dom/ajax-spec.ts
+++ b/spec/observables/dom/ajax-spec.ts
@@ -399,6 +399,9 @@ describe('Observable.ajax', () => {
 
       expect(MockXMLHttpRequest.mostRecent.url).to.equal('/flibbertyJibbet');
       expect(MockXMLHttpRequest.mostRecent.data).to.deep.equal(body);
+      expect(MockXMLHttpRequest.mostRecent.requestHeaders).to.deep.equal({
+        'X-Requested-With': 'XMLHttpRequest',
+      });
     });
 
     it('should not fail when FormData is undefined', () => {
@@ -552,4 +555,3 @@ describe('Observable.ajax', () => {
     });
   });
 });
-
diff --git a/src/observable/dom/AjaxObservable.ts b/src/observable/dom/AjaxObservable.ts
index 87da9f89f1..513cfe1407 100644
--- a/src/observable/dom/AjaxObservable.ts
+++ b/src/observable/dom/AjaxObservable.ts
@@ -198,7 +198,7 @@ export class AjaxSubscriber<T> extends Subscriber<Event> {
     }
 
     // ensure content type is set
-    if (!('Content-Type' in headers)) {
+    if (!('Content-Type' in headers) && !(root.FormData && request.body instanceof root.FormData)) {
       headers['Content-Type'] = 'application/x-www-form-urlencoded; charset=UTF-8';
     }
 
@@ -271,16 +271,18 @@ export class AjaxSubscriber<T> extends Subscriber<Event> {
     }
   }
 
-  private serializeBody(body: any, contentType: string) {
+  private serializeBody(body: any, contentType?: string) {
     if (!body || typeof body === 'string') {
       return body;
     } else if (root.FormData && body instanceof root.FormData) {
       return body;
     }
 
-    const splitIndex = contentType.indexOf(';');
-    if (splitIndex !== -1) {
-      contentType = contentType.substring(0, splitIndex);
+    if (contentType) {
+      const splitIndex = contentType.indexOf(';');
+      if (splitIndex !== -1) {
+        contentType = contentType.substring(0, splitIndex);
+      }
     }
 
     switch (contentType) {
@@ -288,6 +290,8 @@ export class AjaxSubscriber<T> extends Subscriber<Event> {
         return Object.keys(body).map(key => `${encodeURI(key)}=${encodeURI(body[key])}`).join('&');
       case 'application/json':
         return JSON.stringify(body);
+      default:
+        return body;
     }
   }