Use is_iterable as xabbuh suggested

Author: symfonyaml

src/Symfony/Component/Validator/Constraints/ChoiceValidator.php

@@ -55,11 +55,11 @@ public function validate($value, Constraint $constraint)
                 throw new ConstraintDefinitionException('The Choice constraint expects a valid callback.');
             }
             $choices = $choices();
-            if ($choices instanceof \Traversable) {
-                $choices = iterator_to_array($choices);
-            }
             if (!is_array($choices)) {
-                throw new ConstraintDefinitionException(sprintf('The Choice constraint callback "%s" expects to return an iterable, got "%s".', trim($this->formatValue($constraint->callback), '"'), get_debug_type($choices)));
+                if (!is_iterable($choices)) {
+                    throw new ConstraintDefinitionException(sprintf('The Choice constraint callback "%s" expects to return an iterable, got "%s".', trim($this->formatValue($constraint->callback), '"'), get_debug_type($choices)));
+                }
+                $choices = iterator_to_array($choices);
             }
         } else {
             $choices = $constraint->choices;