My first guess would be that your control is not actually on the master page but on a nested page. If so, this won't find it. You would need to do something like:
this.Form.FindControl("txtEmployee_code");
You can find more examples and more ideas here (make sure to look at the comments, since they have other options people have tried):
http://csharporcas.blogspot.com/2006/05/master-page-find-control.html[
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