diff --git a/doctrine-mapping.xsd b/doctrine-mapping.xsd
index c76ae49b0..bb4a1cc39 100644
--- a/doctrine-mapping.xsd
+++ b/doctrine-mapping.xsd
@@ -229,7 +229,7 @@
       <xs:any minOccurs="0" maxOccurs="unbounded" namespace="##other"/>
     </xs:sequence>
     <xs:attribute name="value" type="xs:NMTOKEN" use="required"/>
-    <xs:attribute name="class" type="xs:NMTOKEN" use="required"/>
+    <xs:attribute name="class" type="xs:string" use="required"/>
     <xs:anyAttribute namespace="##other"/>
   </xs:complexType>