I have the same problem and solved it except when the field is blank as follows:

SELECT client
Found=.t.
Locate for client.clientnum=this.value
if !Found()
Found=.f.
endif
If Found
etc., etc.

I have no idea why this works and the other doesn't

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>I have a form that I need to be able to lookup a record from a different file (CLIENT) by 1 of 2 fields in the CLIENT file. The first, which works, is by CLIENT name which is selected from a combo. The combo is bound to the unique CLIENT id and is saved in the new file (LOG).
>
>The second way to locate the client is to key in a CLIENT number (different than the unique ID). I added a textbox to the form to allow possible input of the number. In processing the search of the matching CLIENT by number, I created a VALID method for the textbox and added this code....(the cbClient is the combo for the client name)...
>
>SELECT client
>Locate for client.clientnum=this.value
>if !Found()
> Messagebox("Client Number not found",48,"ERROR")
> return 0
>ELSE
> Thisform.pageframe1.page1.cbClient.Value=client.clientid
> return 1
>ENDIF
>
>
>
>
>I thought this would work and actually, it appears to be a hit or miss. By stepping through the code, the FOUND() returns a .T. even when a matching number is not found.
>
>I also tried to use the index for the CLIENT number and a SEEK() but when I tried to add a SET INDEX TO CLIENTNUM I got an error about table/cursor buffering which through me. (I still need to work on the buffering stuff for this app... right now everything is set for the defaults)
>
>Is there a way to have more than one INDEX available at a time on a form? Would I add multiple instances of the file in the DE and set the ORDER property differently or something similar?
>
>Suggestions?