ERROR: invalid page header in block

Hello..
During a regular query , I got an werd error:

billing=> select sum(duration)/60 from cdr__2006 where
callstarttime<'2006-06-19 00:00:00';
ERROR: invalid page header in block 3527 of relation "cdr__2006_04_22";

Table cdr__2006_04_22 is a child of cdr__2006_04 , which is a child to
cdr__2006.

Can somebody explain this error ? What should I do ?

Thanks
Alex