diff --git a/lib/Doctrine/ORM/Tools/SchemaTool.php b/lib/Doctrine/ORM/Tools/SchemaTool.php
index b8e62739b..5c222be65 100644
--- a/lib/Doctrine/ORM/Tools/SchemaTool.php
+++ b/lib/Doctrine/ORM/Tools/SchemaTool.php
@@ -345,7 +345,7 @@ class SchemaTool
         $discrColumn = $class->discriminatorColumn;
 
         if ( ! isset($discrColumn['type']) ||
-            (strtolower($discrColumn['type']) == 'string' && $discrColumn['length'] === null)
+            (strtolower($discrColumn['type']) == 'string' && ! isset($discrColumn['length']))
         ) {
             $discrColumn['type'] = 'string';
             $discrColumn['length'] = 255;
diff --git a/tests/Doctrine/Tests/ORM/Tools/SchemaToolTest.php b/tests/Doctrine/Tests/ORM/Tools/SchemaToolTest.php
index 2bd9916ce..6cce06e21 100644
--- a/tests/Doctrine/Tests/ORM/Tools/SchemaToolTest.php
+++ b/tests/Doctrine/Tests/ORM/Tools/SchemaToolTest.php
@@ -2,6 +2,9 @@
 
 namespace Doctrine\Tests\ORM\Tools;
 
+use Doctrine\ORM\Mapping\ClassMetadata;
+use Doctrine\ORM\Tools\Event\GenerateSchemaEventArgs;
+use Doctrine\ORM\Tools\Event\GenerateSchemaTableEventArgs;
 use Doctrine\ORM\Tools\SchemaTool;
 use Doctrine\ORM\Tools\ToolEvents;
 use Doctrine\ORM\Tools\Event\GenerateSchemaTableEventArgs;
@@ -138,6 +141,26 @@ class SchemaToolTest extends \Doctrine\Tests\OrmTestCase
         $this->assertTrue($table->hasIndex('primary'));
         $this->assertTrue($table->hasIndex('uniq_hash'));
     }
+
+    public function testSetDiscriminatorColumnWithoutLength()
+    {
+        $em         = $this->_getTestEntityManager();
+        $schemaTool = new SchemaTool($em);
+        $metadata   = $em->getClassMetadata(__NAMESPACE__ . '\\FirstEntity');
+
+        $metadata->setInheritanceType(ClassMetadata::INHERITANCE_TYPE_SINGLE_TABLE);
+        $metadata->setDiscriminatorColumn(['name' => 'discriminator', 'type' => 'string']);
+
+        $schema = $schemaTool->getSchemaFromMetadata([$metadata]);
+
+        $this->assertTrue($schema->hasTable('first_entity'));
+        $table = $schema->getTable('first_entity');
+
+        $this->assertTrue($table->hasColumn('discriminator'));
+        $column = $table->getColumn('discriminator');
+
+        $this->assertEquals(255, $column->getLength());
+    }
 }
 
 /**