oauth_signature invalid while accessing protected user data

Hi,

I have got a problem while requesting the contact list:

I have read the guide of the url http://developer.yahoo.com/oauth/guide/oauth-auth-flow.html.
And got the oauth_token ,oauth_token_secret and xoauth_yahoo_guid
Than I generate a query url to get my contact list, I got an error: signature invalid.

And I have followed the instruction of Appendix A.5.  Accessing Protected Resources

What am I doing wrong? Why a signature invalid error?

Can someone help? Thank you.