Unable to Find CheckBox in ASP.Net DataList on Button Click Event

hi, i have datalist and on button click event i check the checkbox is checked or not for this my code is

  <asp:DataList runat="server" ID="dlstproductsize"> 
<ItemTemplate> 
<input type="checkbox" value="<%#eval("size_id")%>" id="chksize" runat="server" /> &nbsp;&nbsp; <%#Eval("name")%> 
</ItemTemplate> 
</asp:DataList>

 If chkallowsize.Checked = True Then 
	For Each item In dlstproductsize.Items 
		If (CType(item.FindControl("chksize"), HtmlInputCheckBox)).Checked = True Then 
		ocommon.ProductMapingInsert(uniq_id.Text, CType(item.FindControl("chksize"), HtmlInputCheckBox).Value) 
		End If 
	Next 
End If

i get the error like object reference not set to the object plz help me to find the problem?