Grails导出插件奇怪的行为，只在PDF上出错，其他则很好

我将1.6与Grails 2.3.8一起使用，这是我的控制器的列表方法：

def list () {
    println "*** List method called."
    println "*** Params: " + params
    if(!params.max) params.max = 10

    if(params?.exportFormat && params.exportFormat != "html"){
        response.contentType = grailsApplication.config.grails.mime.types[params.exportFormat]
        response.setHeader("Content-disposition", "attachment; filename=chairs.${params.extension}")

        List fields = ["constructionMaterials"]
        Map labels = ["constructionMaterials": "Construction Materials"]
        // Formatter closure
        def upperCase = { domain, value ->
            return value.toUpperCase()
        }

        Map formatters = [constructionMaterials: upperCase]
        Map parameters = [constructionMaterials: "Cool Chairs", "column.widths": [0.2, 0.3, 0.5]]
        println "*** Does it make it here?"
        exportService.export(params.exportFormat, response.outputStream,Chair.list(params), fields, labels, formatters, parameters)
    }

    [ chairInstanceList: Chair.list( params ) ]
}

当我点击CSV和XML时，它工作正常，但当我点击PDF时，我得到了这个错误

| Error 2014-06-26 01:47:49,139 [http-bio-8080-exec-8] ERROR [/demoApp].[default]  - Servlet.service() for servlet [default] in context with path [/demoApp] threw exception [org.springframework.web.util.NestedServletException: Request processing failed; nested exception is groovy.lang.MissingMethodException: No signature of method: static java.lang.Math.max() is applicable for argument types: (java.lang.Integer, null) values: [4, null]
Possible solutions: max(int, int), max(double, double), max(float, float), max(long, long), min(int, int), wait()] with root cause
Message: No signature of method: static java.lang.Math.max() is applicable for argument types: (java.lang.Integer, null) values: [4, null]
Possible solutions: max(int, int), max(double, double), max(float, float), max(long, long), min(int, int), wait()
    Line | Method
->>  251 | doAppend              in org.apache.log4j.AppenderSkeleton
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
|     66 | appendLoopOnAppenders in org.apache.log4j.helpers.AppenderAttachableImpl
|    206 | callAppenders . . . . in org.apache.log4j.Category
|    391 | forcedLog             in     ''
|    856 | log . . . . . . . . . in     ''
|    192 | logMessage            in org.slf4j.impl.GrailsLog4jLoggerAdapter
|    167 | error . . . . . . . . in     ''
|    213 | error                 in org.apache.commons.logging.impl.SLF4JLog
|    198 | doFilter . . . . . .  in grails.plugin.cache.web.filter.PageFragmentCachingFilter
|     63 | doFilter              in grails.plugin.cache.web.filter.AbstractFilter
|   1145 | runWorker . . . . . . in java.util.concurrent.ThreadPoolExecutor
|    615 | run                   in java.util.concurrent.ThreadPoolExecutor$Worker
^    744 | run . . . . . . . . . in java.lang.Thread

为什么它可以导出两种格式，但在PDF上会出错

更新：好吧，这很奇怪，但我认为当我有超过4个域实例时，PDF选项最终会起作用。当该数字等于或低于该数字时，则不适用。有人能解释一下吗？当它最终起作用时，我得到的是文件。并且是我得到的等效XML文件