ASP Type mismatch error with SELECT...FOR UPDATE statement

ASP error number 13 - Type mismatch with SELECT...FOR UPDATE statement

I got ASP error number 13 when I use the SELECT...FOR UPDATE statement
as below.
However, if I use SELECT statement without FOR UPDATE, it is fine and
no error.
I also tried Set objRs = objConn.Execute("SELECT * FROM EMP UPDATE OF
EMPNO"), but it still couldn't help.

any ideas? I tried to search in the web but couldn't find similar
problem. Is it because
the setting problems?


Here's the code fragment:
========================
On Error Resume Next
Set objConn = Server.CreateObject("ADODB.Connection")
objConn.Open strConnectionString
Set objRs = Server.CreateObject("ADODB.RecordSet")
objRs.CursorLocation = adUseClient
objRs.open "SELECT * FROM EMP UPDATE OF EMPNO", objConn,
adOpenForwardOnly, adLockBatchOptimistic
//etc...
ErrHndl:
response.write Err.Source & "
"
response.write "Error number " & err.number & "
"
response.write "Error description " & err.description & "
"
response.end


Errors:
=======================
Microsoft VBScript runtime error
Error number 13
Error description Type mismatch


Please advise. thanks a lot!!