A question

My postgresql version is 7.2.1
I have create a function:
create function returnset(varchar) returns setofcomputers
as '
select * from computer where hostname like ''%''+$1+''%'' '
language 'plpgsql';

But when I select returnset('aaa') ,it returns:

An error has occured in pgAdmin
II:frmSQLInput.cmdExecute_Click:
Number:-2147467259
Description:ERROR:fmgr_info:function 0:cache lookup failed

Please tell me what means!
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