diff --git a/lib/Doctrine/ORM/Mapping/Driver/AnnotationDriver.php b/lib/Doctrine/ORM/Mapping/Driver/AnnotationDriver.php
index 1f8599657..9dd64bbd3 100644
--- a/lib/Doctrine/ORM/Mapping/Driver/AnnotationDriver.php
+++ b/lib/Doctrine/ORM/Mapping/Driver/AnnotationDriver.php
@@ -242,8 +242,8 @@ class AnnotationDriver extends AbstractAnnotationDriver
 
                     $metadata->setDiscriminatorColumn(array(
                         'name'             => $discrColumnAnnot->name,
-                        'type'             => $discrColumnAnnot->type,
-                        'length'           => $discrColumnAnnot->length,
+                        'type'             => $discrColumnAnnot->type ?: 'string',
+                        'length'           => $discrColumnAnnot->length ?: 255,
                         'columnDefinition' => $discrColumnAnnot->columnDefinition,
                     ));
                 } else {
diff --git a/lib/Doctrine/ORM/Mapping/Driver/XmlDriver.php b/lib/Doctrine/ORM/Mapping/Driver/XmlDriver.php
index da090daa4..00de4f2ea 100644
--- a/lib/Doctrine/ORM/Mapping/Driver/XmlDriver.php
+++ b/lib/Doctrine/ORM/Mapping/Driver/XmlDriver.php
@@ -165,8 +165,8 @@ class XmlDriver extends FileDriver
                     $discrColumn = $xmlRoot->{'discriminator-column'};
                     $metadata->setDiscriminatorColumn(array(
                         'name' => isset($discrColumn['name']) ? (string) $discrColumn['name'] : null,
-                        'type' => isset($discrColumn['type']) ? (string) $discrColumn['type'] : null,
-                        'length' => isset($discrColumn['length']) ? (string) $discrColumn['length'] : null,
+                        'type' => isset($discrColumn['type']) ? (string) $discrColumn['type'] : 'string',
+                        'length' => isset($discrColumn['length']) ? (string) $discrColumn['length'] : 255,
                         'columnDefinition' => isset($discrColumn['column-definition']) ? (string) $discrColumn['column-definition'] : null
                     ));
                 } else {
diff --git a/lib/Doctrine/ORM/Mapping/Driver/YamlDriver.php b/lib/Doctrine/ORM/Mapping/Driver/YamlDriver.php
index 1dee969d7..fe406785f 100644
--- a/lib/Doctrine/ORM/Mapping/Driver/YamlDriver.php
+++ b/lib/Doctrine/ORM/Mapping/Driver/YamlDriver.php
@@ -176,8 +176,8 @@ class YamlDriver extends FileDriver
                     $discrColumn = $element['discriminatorColumn'];
                     $metadata->setDiscriminatorColumn(array(
                         'name' => isset($discrColumn['name']) ? (string) $discrColumn['name'] : null,
-                        'type' => isset($discrColumn['type']) ? (string) $discrColumn['type'] : null,
-                        'length' => isset($discrColumn['length']) ? (string) $discrColumn['length'] : null,
+                        'type' => isset($discrColumn['type']) ? (string) $discrColumn['type'] : 'string',
+                        'length' => isset($discrColumn['length']) ? (string) $discrColumn['length'] : 255,
                         'columnDefinition' => isset($discrColumn['columnDefinition']) ? (string) $discrColumn['columnDefinition'] : null
                     ));
                 } else {