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>