diff --git a/vle_specific.php b/vle_specific.php
index fbd4e7ff261399e0bafa532b78044a6017d579c5..d62e8172170c4e15db9a25b0747d2729a9bd7f47 100644
--- a/vle_specific.php
+++ b/vle_specific.php
@@ -204,7 +204,7 @@ function stack_get_mathjax_url(): string {
             $urlstring = substr($mathjaxconfigurl, 0, $questionpos);
             parse_str($querystring, $queryparams);
             $queryparams = array_merge(['config' => 'TeX-AMS-MML_HTMLorMML'], $queryparams);
-            $querystring = http_build_query($queryparams, null, '&', PHP_QUERY_RFC3986);
+            $querystring = http_build_query($queryparams, '', '&', PHP_QUERY_RFC3986);
             $url = $urlstring . '?' . $querystring;
         } else {
             $url = $mathjaxconfigurl . '?config=TeX-AMS-MML_HTMLorMML';