Trying to use umbraco.presentation.umbracoBase.library.member that is already present in restExtensions.config in a Umbraco 4.7.1.Beta installation

<ext assembly="umbraco" type="umbraco.presentation.umbracoBase.library.member" alias="currentMember">
    <permission method="login" allowAll="true" />
    <permission method="logout" allowAll="true" />
    <permission method="id" allowAll="true" />
    <permission method="data" allowAll="true" />
    <permission method="setProperty" allowAll="false" />
  </ext>

Trying to invoke method using URL below

http://localhost:50799/base/currentMember/login/mtv/password.aspx

Keeping getting below error message

Server Error in '/' Application.
Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 
Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.
Source Error: 
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
Stack Trace: 
[NullReferenceException: Object reference not set to an instance of an object.]
   umbraco.presentation.umbracobase.restExtension..ctor(String extensionAlias, String methodName) +1337
   umbraco.presentation.umbracobase.requestModule.httpApp_PostAcquireRequestState(Object sender, EventArgs e) +712
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +148
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75
Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.237

Any ideas?