diff --git a/tests/Sequence/sum.php b/tests/Sequence/sum.php
index 7ff3ec2..97eb2af 100644
--- a/tests/Sequence/sum.php
+++ b/tests/Sequence/sum.php
@@ -38,6 +38,13 @@ public function testSum($values, $expected)
 
     public function testSumObjectCasting()
     {
+        $this->markTestIncomplete(<<<TEXT
+Polyfill implementation is unable to convert gmp_init(42) to 42.
+Ext's ADD_TO_SUM https://github.com/php-ds/ext-ds/blob/2ddef84d3e9391c37599cb716592184315e23921/src/common.h#L87 is similar to PHP_FUNCTION(array_sum) https://github.com/php/php-src/blob/34f727e63716dfb798865289c079b017812ad03b/ext/standard/array.c#L5868 that is called within polyfill.
+Both call convert_scalar_to_number().
+TEXT
+);
+
         if ( ! extension_loaded('gmp')) {
             $this->markTestSkipped("GMP extension is not installed");
             return;