Index: Zend/zend_interfaces.c
===================================================================
--- Zend/zend_interfaces.c	(revision 289211)
+++ Zend/zend_interfaces.c	(working copy)
@@ -344,6 +344,10 @@
 			if (class_type->num_interfaces) {
 				for (i = 0; i < class_type->num_interfaces; i++) {
 					if (class_type->interfaces[i] == zend_ce_iterator) {
+						zend_error(E_CORE_ERROR, "Class %s cannot implement both %s and %s at the same time.",
+									class_type->name,
+									interface->name,
+									zend_ce_iterator->name);
 						return FAILURE;
 					}
 					if (class_type->interfaces[i] == zend_ce_traversable) {