Fallback to JSON renderer if no valid format is given

core/API/Request.php

@@ -215,6 +215,8 @@ private function sanitizeRequest()
      */
     public function process()
     {
+        $shouldReloadAuth = false;
+
         try {
             ++self::$nestedApiInvocationCount;
 
@@ -233,7 +235,6 @@ public function process()
             $corsHandler->handle();
 
             $tokenAuth = Common::getRequestVar('token_auth', '', 'string', $this->request);
-            $shouldReloadAuth = false;
 
             // IP check is needed here as we cannot listen to API.Request.authenticate as it would then not return proper API format response.
             // We can also not do it by listening to API.Request.dispatch as by then the user is already authenticated and we want to make sure
@@ -276,6 +277,10 @@ public function process()
                 'ignoreInScreenWriter' => true,
             ]);
 
+            if (empty($response)) {
+               $response = new ResponseBuilder('console', $this->request);
+            }
+
             $toReturn = $response->getResponseException($e);
         } finally {
             --self::$nestedApiInvocationCount;

tests/PHPUnit/System/ApiGetReportMetadataTest.php

@@ -75,6 +75,14 @@ public function getApiForTesting()
             array('Actions.getPageTitles', array('idSite'     => $idSite, 'date' => $dateTime,
                                                  'testSuffix' => '_pageTitleZeroString')),
 
+            // Test w/ no format, should default to format=json
+            ['Actions.getPageTitles', [
+                'idSite'     => $idSite,
+                'date' => $dateTime,
+                'testSuffix' => '_defaultFormatValue',
+                'format' => 'asldjkf',
+            ]],
+
             // test php renderer w/ array data
             array('API.getDefaultMetricTranslations', array('idSite' => $idSite, 'date' => $dateTime,
                                                             'format' => 'php', 'testSuffix' => '_phpRenderer')),

tests/PHPUnit/System/expected/test_apiGetReportMetadata_defaultFormatValue__Actions.getPageTitles_day.asldjkf

@@ -0,0 +1,3 @@
+Error: Renderer format 'asldjkf' not valid. Try any of the following instead: console, csv, html, json, json2, original, php, rss, tsv, xml.
+
+ --> To temporarily debug this error further, set const PIWIK_PRINT_ERROR_BACKTRACE=true; in index.php