Error when trying to do "clean up" with v1.7.1 & 1.7.2

Hi Experts,
  
 I get the following error message when trying to do "clean up".
  
 Thanks,
 Shi Liu
  
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Subversion Exception!
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Subversion encountered a serious problem.
Please take the time to report this on the Subversion mailing list
with as much information as possible about what
you were trying to do.
But please first search the mailing list archives for the error message
to avoid reporting the same problem repeatedly.
You can find the mailing list archives at
http://subversion.apache.org/mailing-lists.html
 Subversion reported the following
(you can copy the content of this dialog
to the clipboard using Ctrl-C):
 In file
 'D:\Development\SVN\Releases\TortoiseSVN-1.7.2\ext\subversion\subversion\libsvn_wc\workqueue.c'
 line 673: assertion failed (checksum != NULL)
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OK
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