diff --git a/lib/Doctrine/ORM/Mapping/Driver/XmlDriver.php b/lib/Doctrine/ORM/Mapping/Driver/XmlDriver.php
index b676ca8da..fd6bf081a 100644
--- a/lib/Doctrine/ORM/Mapping/Driver/XmlDriver.php
+++ b/lib/Doctrine/ORM/Mapping/Driver/XmlDriver.php
@@ -89,7 +89,7 @@ class XmlDriver extends AbstractFileDriver
         if (isset($xmlRoot['schema'])) {
             $metadata->table['schema'] = (string)$xmlRoot['schema'];
         }*/
-        
+
         if (isset($xmlRoot['inheritance-type'])) {
             $inheritanceType = (string)$xmlRoot['inheritance-type'];
             $metadata->setInheritanceType(constant('Doctrine\ORM\Mapping\ClassMetadata::INHERITANCE_TYPE_' . $inheritanceType));
@@ -327,6 +327,8 @@ class XmlDriver extends AbstractFileDriver
 
                 if (isset($oneToManyElement['index-by'])) {
                     $mapping['indexBy'] = (string)$oneToManyElement['index-by'];
+                } else if (isset($oneToManyElement->{'index-by'})) {
+                    throw new \InvalidArgumentException("<index-by /> is not a valid tag");
                 }
 
                 $metadata->mapOneToMany($mapping);
@@ -432,8 +434,10 @@ class XmlDriver extends AbstractFileDriver
                     $mapping['orderBy'] = $orderBy;
                 }
 
-                if (isset($manyToManyElement->{'index-by'})) {
-                    $mapping['indexBy'] = (string)$manyToManyElement->{'index-by'};
+                if (isset($manyToManyElement['index-by'])) {
+                    $mapping['indexBy'] = (string)$manyToManyElement['index-by'];
+                } else if (isset($manyToManyElement->{'index-by'})) {
+                    throw new \InvalidArgumentException("<index-by /> is not a valid tag");
                 }
 
                 $metadata->mapManyToMany($mapping);