diff --git a/classes/external/run_in_sandbox.php b/classes/external/run_in_sandbox.php
index 7e2475af4..82738bccf 100644
--- a/classes/external/run_in_sandbox.php
+++ b/classes/external/run_in_sandbox.php
@@ -111,9 +111,9 @@ public static function execute($contextid, $sourcecode, $language='python3',
             throw new qtype_coderunner_exception(get_string('wsnoaccess', 'qtype_coderunner'));
         }
 
-        $sandbox = qtype_coderunner_sandbox::get_best_sandbox($language);
+        $sandbox = qtype_coderunner_sandbox::get_best_sandbox($language, true);
         if ($sandbox === null) {
-            throw new qtype_coderunner_exception("Language {$language} is not available on this system");
+            throw new qtype_coderunner_exception(get_string('wsnolanguage', 'qtype_coderunner', $language));
         }
 
         if (get_config('qtype_coderunner', 'wsloggingenabled')) {
@@ -139,8 +139,13 @@ public static function execute($contextid, $sourcecode, $language='python3',
         }
 
         try {
-            $filesarray = $files ? json_decode($files, true) : null;
+            $filesarray = $files ? json_decode($files, true) : array();
             $paramsarray = $params ? json_decode($params, true) : array();
+            
+            // Throws error for incorrect JSON formatting
+            if ($filesarray === null || $paramsarray === null) {
+                throw new qtype_coderunner_exception(get_string('wsbadjson', 'qtype_coderunner'));
+            }
             $maxcputime = intval(get_config('qtype_coderunner', 'wsmaxcputime'));  // Limit CPU time through this service.
             if (isset($paramsarray['cputime'])) {
                 $paramsarray['cputime'] = min($paramsarray['cputime'], $maxcputime);
diff --git a/lang/en/qtype_coderunner.php b/lang/en/qtype_coderunner.php
index d800c3bcf..103c6175d 100644
--- a/lang/en/qtype_coderunner.php
+++ b/lang/en/qtype_coderunner.php
@@ -1221,14 +1221,16 @@ function should be applied, e.g. <code>{{STUDENT_ANSWER | e(\'py\')}}</code> is
 $string['validateonsave'] = 'Validate on save';
 
 $string['wrongnumberofformats'] = 'Wrong number of test results column formats. Expected {$a->expected}, got {$a->got}';
+$string['wsbadjson'] = 'params and file parameters must be blank or a valid JSON record';
 $string['wsdisabled'] = 'Sandbox web service disabled. Talk to a sysadmin';
 $string['wsloggingenable'] = 'Log sandbox web service usage';
 $string['wsloggingenable_desc'] = 'If this option is checked, every code execution via the sandbox web service will be logged. This option must be enabled if user rate throttling is to work.';
-$string['wsnoaccess'] = 'Only logged-in non-guest users can access this functionality';
 $string['wsmaxcputime'] = 'Max CPU time (secs)';
 $string['wsmaxcputime_desc'] = 'Limits the maximum CPU time that a web service job can use, even if it explicitly sets the CPU time sandbox parameter.';
 $string['wsmaxhourlyrate'] = 'Max hourly rate of submissions';
 $string['wsmaxhourlyrate_desc'] = 'If a user attempts to exceed this rate of submissions in any given hour their submissions will be disallowed. 0 for no rate throttling. Requires that logging of web service usage be enabled.';
+$string['wsnoaccess'] = 'Only logged-in non-guest users can access this functionality';
+$string['wsnolanguage'] = 'Language "{$a}" is not known';
 $string['wssubmissionrateexceeded'] = 'You have exceeded the maximum hourly \'Try it!\' submission rate. Request denied.';
 
 $string['xmlcoderunnerformaterror'] = 'XML format error in coderunner question';