Return undefined.

README.md

@@ -85,7 +85,7 @@ d3.image("https://example.com/test.png", {crossOrigin: "anonymous"}).then(functi
 
 <a name="json" href="#json">#</a> d3.<b>json</b>(<i>input</i>[, <i>init</i>]) [<>](https://github.com/d3/d3-fetch/blob/master/src/json.js "Source")
 
-Fetches the [JSON](http://json.org) file at the specified *input* URL. If *init* is specified, it is passed along to the underlying call to [fetch](https://fetch.spec.whatwg.org/#fetch-method); see [RequestInit](https://fetch.spec.whatwg.org/#requestinit) for allowed fields.
+Fetches the [JSON](http://json.org) file at the specified *input* URL. If *init* is specified, it is passed along to the underlying call to [fetch](https://fetch.spec.whatwg.org/#fetch-method); see [RequestInit](https://fetch.spec.whatwg.org/#requestinit) for allowed fields. If the server returns a status code of [204 No Content](https://developer.mozilla.org/docs/Web/HTTP/Status/204) or [205 Reset Content](https://developer.mozilla.org/docs/Web/HTTP/Status/205), the promise resolves to `undefined`.
 
 <a name="svg" href="#svg">#</a> d3.<b>svg</b>(<i>input</i>[, <i>init</i>]) [<>](https://github.com/d3/d3-fetch/blob/master/src/xml.js "Source")
 

src/json.js

@@ -1,8 +1,6 @@
 function responseJson(response) {
   if (!response.ok) throw new Error(response.status + " " + response.statusText);
-  if (response.status === 204 || response.status === 205) {
-    return null;
-  }
+  if (response.status === 204 || response.status === 205) return;
   return response.json();
 }