how to clear binded input tag on button click

hello
i m using the input tag in my program and bind it like:

<input type="text" id="txtPCC" name="txtPCC" value="<%=ds.Tables["abc"].Rows[0].ItemArray[6].ToString()%>" onchange="checkInput(this,'<%=txtPCC_DUE.ClientID%>')" />

now the problem is,when i click the add button it gives error System.NullReferenceException: Object reference not set to an instance of an object.

i want input be blank when i click the add button... how can i?????/

please help