Struts2返回json报错，求解
使用的是struts2-json-plugin-2.3.1.jar
我的Action如下：

public String login(){

		Account _account = new Account();

		_account.setUserName(userName);

		_account.setPassword(password);

		account = accountService.getAccountByUserName(_account);

		if(account != null){

			success = true;

			return "json";

		}else{

			errorMsg = "登录失败!";

			return ERROR;

		}

	}

struts.xml配置如下：

<package name="json" extends ="json-default">

		<action name="loginAction" class="com.bactryki.action.AccountAction">

			<result type="json">

            	<param name="includeProperties">success,account,errorMsg</param>

        	</result>

        	<result name="error">/error.jsp</result>

		</action>

	</package>

执行报错如下，请各位指点一下，拜托，在线等！
No result defined for action com.bactryki.action.AccountAction and result json
at com.opensymphony.xwork2.DefaultActionInvocation.executeResult(DefaultActionInvocation.java:376)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:278)
at org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:256)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:176)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:265)
at org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:138)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:211)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)