freemarker list size问题

取得list的长度：${fields?size}。 

而使用判断其长度的时候， 

 <#if fields?size>0 >
 </#if>

竟抛出错了，说不是boolean类型， 

 Java backtrace for programmers:
 ----------
 freemarker.core.NonBooleanException: Error on line 132, column 22 in viewEntity_js.ftl
 Expecting a boolean (true/false) expression here
 Expression fields?size does not evaluate to true/false
 it is an instance of freemarker.template.SimpleNumber
          at freemarker.core.Expression.isTrue(Expression.java:150)

 <#if (fields?size>0) >
 </#if>