WebLogic 14C Custom Identity Asserter - Unsupported Token Type Error

Currently I am implementing a custom identity asserter in WebLogic 14C environment. I have developed following components. An AuthenticationProvider.xml (which defines MBean Definition File elements)

<MBeanAttribute
  Name          = "ProviderClassName"
  Type          = "java.lang.String"  Writeable     = "false"
  Default       = "&quot;com.test.authentication.CustomAuthenticationProvider&quot;"  />    
<MBeanAttribute
   Name = "SupportedTypes"
  Type = "java.lang.String[]"
  Writeable = "true"
  Default = "new String[] {&quot;CustomToken&quot;}"  />  
<MBeanAttribute
   Name= "ActiveTypes"
  Type= "java.lang.String[]"
  Writeable = "true"
  Default = "new String[] {&quot;CustomToken&quot;}"  />

I have also class that implements interfaces AuthenticationProviderV2, IdentityAsserterV2

public class CustomAuthenticationProvider implements AuthenticationProviderV2, IdentityAsserterV2 {
  @Overridepublic 
  IdentityAsserterV2 getIdentityAsserter() {    
  return this;
}
public PrincipalValidator getPrincipalValidator() {    
  return new PrincipalValidatorImpl();
}
@Override
public CallbackHandler assertIdentity(String type, Object token, ContextHandler context) throws IdentityAssertionException  {
      System.out.println("NSPIRESOracleLoginModule.assertIdentity");
      System.out.println("\tType\t\t= "  + type);
      System.out.println("\tToken\t\t= " + token);      
      return null;   
}