diff --git a/lib/ajax/service.php b/lib/ajax/service.php
index 5437f1a4b5d..43ce3f70438 100644
--- a/lib/ajax/service.php
+++ b/lib/ajax/service.php
@@ -36,7 +36,12 @@ $rawjson = file_get_contents('php://input');
 
 $requests = json_decode($rawjson, true);
 if ($requests === null) {
-    $lasterror = json_last_error_msg();
+    if (function_exists('json_last_error_msg')) {
+        $lasterror = json_last_error_msg();
+    } else {
+        // Fall back to numeric error for older PHP version.
+        $lasterror = json_last_error();
+    }
     throw new coding_exception('Invalid json in request: ' . $lasterror);
 }
 $responses = array();