diff --git a/packages/adapter/addon/json-api.js b/packages/adapter/addon/json-api.js
index ff9de5bbbe2..d86da31278a 100644
--- a/packages/adapter/addon/json-api.js
+++ b/packages/adapter/addon/json-api.js
@@ -143,7 +143,7 @@ import { pluralize } from 'ember-inflector';
 const JSONAPIAdapter = RESTAdapter.extend({
   defaultSerializer: '-json-api',
 
-  defaultContentType: 'application/vnd.api+json',
+  _defaultContentType: 'application/vnd.api+json',
 
   /**
     @method ajaxOptions
diff --git a/packages/adapter/addon/rest.js b/packages/adapter/addon/rest.js
index 7583c3365ed..16d64b6faa7 100644
--- a/packages/adapter/addon/rest.js
+++ b/packages/adapter/addon/rest.js
@@ -291,7 +291,7 @@ const hasNajax = typeof najax !== 'undefined';
 const RESTAdapter = Adapter.extend(BuildURLMixin, {
   defaultSerializer: '-rest',
 
-  defaultContentType: 'application/json; charset=utf-8',
+  _defaultContentType: 'application/json; charset=utf-8',
 
   fastboot: computed(function() {
     return getOwner(this).lookup('service:fastboot');
@@ -1095,7 +1095,7 @@ const RESTAdapter = Adapter.extend(BuildURLMixin, {
       options.headers = {};
     }
 
-    let contentType = options.contentType || this.defaultContentType;
+    let contentType = options.contentType || this._defaultContentType;
 
     if (get(this, 'useFetch')) {
       if (options.data && options.type !== 'GET') {
@@ -1105,6 +1105,8 @@ const RESTAdapter = Adapter.extend(BuildURLMixin, {
       }
       options = fetchOptions(options, this);
     } else {
+      // GET requests without a body should not have a content-type header
+      // and may be unexpected by a server
       if (options.data && options.type !== 'GET') {
         options = assign(options, { contentType });
       }