How do you include Xml Docs for a class library in a NuGet package?

I am creating a NuGet package for a C# class library, and I would like to include generated Xml Documentation with the library. This is my nuspec file:

<?xml version="1.0" encoding="utf-8"?>
<package>
  <metadata>
    <id>MyLibrary</id>
    <version>1.0.0.0</version>
    <authors>John Nelson</authors>
    <language>en-US</language>
    <description>A C# class library</description>
  </metadata>
  <files>
    <file src="..\..\build\MyLibrary.dll" target="lib\Net40" />
    <file src="..\..\build\MyLibrary.xml" target="lib\Net40" />
  </files>
</package>

When I build the package with this command:

nuget pack MyLibrary.nuspec

It generates an error. If I remove the line:

<file src="..\..\build\MyLibrary.xml" target="lib\Net40" />

NuGet.exe successfully creates the nupkg. I can even unzip the package, and verify that the contents are correct. What am I doing wrong? Should the xml file go into a different target directory?