numBackups failed when using dot into pattern

[crochut]

Hi,
i using this configuration

{
appenders: {
console: { type: "stdout" },
file: {
type: "dateFile",
filename: conf.directory + '/' +conf.filePattern,
pattern:'yyyy.MM.dd.hh.mm',
numBackups : 1,
},
},
categories: {
default: { appenders: ["file"], level: conf.logLevel },
}
}

And the rolling is ok but the numBackups doesn't work.

If i replace '.' by '-' into filename, it works fine.

pattern:'yyyy-MM-dd-hh-mm',