The exception should be thrown if an object doesn't implement Traversable AND doesn't implement Countable, not when it doesn't implement Traversable but DOES implement Countable

Author: Robin Kanters

Question/Question.php

@@ -140,7 +140,7 @@ public function setAutocompleterValues($values)
         }
 
         if (null !== $values && !is_array($values)) {
-            if (!$values instanceof \Traversable || $values instanceof \Countable) {
+            if (!$values instanceof \Traversable || !$values instanceof \Countable) {
                 throw new \InvalidArgumentException('Autocompleter values can be either an array, `null` or an object implementing both `Countable` and `Traversable` interfaces.');
             }
         }