Geting Error When Accessing RadioButtonList From Javascript That Exist In A Page That Have Master Page?

i have a asp.net page that have its master page and it contain RadioButtonList1 and i try to do thefollwing

<script type="text/javascript">

var radioButtonList = document.getElementById('<%=RadioButtonList1.ClientID%>');
if(radioButtonList[0].checked)
document.getElementById("_secondTR").style.display = "block";
else if (radioButtonList[1].checked )
document.getElementById("_secondTR").style.display = "none";
}
</script>
<table style="width: 100%">
<tr id="Tr1">
<td>
<asp:RadioButtonList ID="RadioButtonList1" runat="server" BackColor="#FFCC99"
RepeatDirection="Horizontal" Width="117px" onclick="ShowHide()">
<asp:ListItem Value="1">Yes</asp:ListItem>
<asp:ListItem Value="0">No</asp:ListItem>
</asp:RadioButtonList>
</td>
</tr>
<tr id="_secondTR" runat="server" style="display: none">
<td>
<asp:RadioButton ID="Five" runat="server" GroupName="1" BackColor="#669999" />
<asp:RadioButton ID="Four" runat="server" GroupName="1" CausesValidation="True" BackColor="#669999" />
</td>
</tr>
</table>

i can't get RadioButtonList1 from java script ...