关于PSTL的多分枝判断的语句问题

老师，我跟着课件中的操作，界面却显示 500的错误，当我把多分枝的语句删去，又正常的显示，代码经过反复的检查，确认没有问题，不知道该如何的操作

jsp

<!-- choose  when  otherwise -->

<c:choose>

<c:when test="${grade=='D' }">

<h1>此语句被执行！</h1>

</c:when>

</c:choose>

HTTP Status 500 – Internal Server Error

Type Exception Report

Message An exception occurred processing JSP page [/core.jsp] at line [23]

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

org.apache.jasper.JasperException: An exception occurred processing JSP page [/core.jsp] at line [23]

20:
21: <!-- choose  when  otherwise -->
22: <c:choose>
23: <c:when test="${grade=='D' }">
24: <h1>姝よ鍙ヨ鎵ц锛�</h1>
25: </c:when>
26: </c:choose>


Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:617)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:514)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:386)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:330)
javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
com.imooc.jstl.JstlServlet.doGet(JstlServlet.java:31)
javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

Root Cause

javax.el.ELException: Cannot convert [D] of type [class java.lang.String] to [class java.lang.Long]
org.apache.el.lang.ELSupport.coerceToNumber(ELSupport.java:400)
org.apache.el.lang.ELSupport.coerceToNumber(ELSupport.java:379)
org.apache.el.lang.ELSupport.equals(ELSupport.java:173)
org.apache.el.parser.AstEqual.getValue(AstEqual.java:39)
org.apache.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:190)
org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(PageContextImpl.java:944)
org.apache.jsp.core_jsp._jspx_meth_c_005fwhen_005f0(core_jsp.java:290)
org.apache.jsp.core_jsp._jspx_meth_c_005fchoose_005f0(core_jsp.java:258)
org.apache.jsp.core_jsp._jspService(core_jsp.java:150)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:476)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:386)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:330)
javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
com.imooc.jstl.JstlServlet.doGet(JstlServlet.java:31)
javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

Note The full stack trace of the root cause is available in the server logs.