diff --git a/master/buildbot/test/unit/test_www_rest.py b/master/buildbot/test/unit/test_www_rest.py
index d253357b257..819e0f65278 100644
--- a/master/buildbot/test/unit/test_www_rest.py
+++ b/master/buildbot/test/unit/test_www_rest.py
@@ -169,6 +169,12 @@ def test_cors_origin_mismatch(self):
         yield self.render_resource(self.rsrc, '/', origin='h://bad')
         self.assertNotOk('invalid origin')
 
+    @defer.inlineCallbacks
+    def test_cors_origin_mismatch_post(self):
+        yield self.render_resource(self.rsrc, '/', method='POST', origin='h://bad')
+        self.assertRequest(content=json.dumps({'error': {'message': 'invalid origin'}}),
+                           responseCode=400)
+
     @defer.inlineCallbacks
     def test_cors_origin_preflight_match_GET(self):
         yield self.render_resource(self.rsrc, '/',
diff --git a/master/buildbot/www/rest.py b/master/buildbot/www/rest.py
index a432a88d31d..504a133aa89 100644
--- a/master/buildbot/www/rest.py
+++ b/master/buildbot/www/rest.py
@@ -426,7 +426,11 @@ def writeError(msg, errcode=400):
                 log.msg("HTTP error: %s" % (msg,))
             request.setResponseCode(errcode)
             request.setHeader('content-type', 'text/plain; charset=utf-8')
-            request.write(json.dumps(dict(error=msg)))
+            if request.method == 'POST':
+                # jsonRPC callers want the error message in error.message
+                request.write(json.dumps(dict(error=dict(message=msg))))
+            else:
+                request.write(json.dumps(dict(error=msg)))
             request.finish()
         return self.asyncRenderHelper(request, self.asyncRender, writeError)