how to get rid of there error "C:\Program Files (x86)\MSBuild\12.0\bin\Microsoft.Common.CurrentVersion.targets(4216,5): error MSB3541: Files has invalid value "". Illegal characters in path."

when double clicked on the message, here is what I got
cursor on the first < below
    <FindUnderPath Path="$(OutDir)" Files="@(_CleanOrphanFileWrites)">
      <Output TaskParameter="InPath" ItemName="_CleanOrphanFileWritesInOutput"/>
    </FindUnderPath>

how do I find the culprit?
thank you fror your time and effort.