Use Ajax Calendar with Master Page in VS2008

I am using Master Page in my Project and the problem is in using ajax calendar exterder in my content page,

if i use calendar extender it gives the error like:

Object reference not set to an instance of an object. 
Descrip开发者_Go百科tion: 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.  

Looking forward for ur valuable solution........

Thnx in Advance, Supriya

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There Must be code behind error. Check error about assigning values. You may need to cast the value.