jsp编译错误

inputNumber.jsp
<%@ page contentType="text/html;Charset=GB2312" %><%@ page language="java" pageEncoding="UTF-8" %>
<%@ taglib tagdir="/WEB-INF/tags" prefix="computer"%>

<html><body bgcolor=cycan><font size=3><FORM action="" method=post name=jisuan >
<INPUT type="text" name="Number1" >
<INPUT type="text" name="Number2">
<select name="xiala">
<option value="1">+</option>
<option value="2">-</option>
<option value="3">*</option>
<option value="4">/</option>
</select>
</FORM>
<%
String Number1=request.getParameter("Number1");
String Number2=request.getParameter("Number2");
String yunsuanfu=request.getParameter("xiala");
%>
<computer:MathComputer operator1="<%=Number1%>" operator2="<%=Number2%>" oyunsuanfu="<%=yunsuanfu%>"/>
<P>计算结果为：</P>
<%=Res%>
</font>
</body>
</html>

MathComputer.tag
<%@ tag language="java" pageEncoding="GB2312"%>
<%@ tag import = "java.lang.*" %>
<%@attribute name="operator1" required="true"%>
<%@attribute name="operator2" required="true"%>
<%@attribute name="oyunsuanfu" required="true"%>
<%@ variable name-given="Res" variable-class="java.lang.Double" scope="AT_END" %>

<%double a=Double.parseDouble(operator1);
double b=Double.parseDouble(operator2);
int c=Integer.parseInt(oyunsuanfu);
double res=0;
switch(c){
case 1:
res=a+b;

break;
case 2:
res=a-b;

break;
case 3:
res=a*b;

break;
case 4:
res=a/b;

break;
default:
out.println("error");
}
jspContext.setAttribute("Res",new Double(res));

%>

报错时说inputNumber.jsp里的<computer:MathComputer operator1="<%=Number1%>" operator2="<%=Number2%>" oyunsuanfu="<%=yunsuanfu%>"/>这一段出错
错误：javax.servlet.ServletException: javax.servlet.jsp.JspException: java.lang.NullPointerExceptio