nuspec contentFiles element invalid in metadata

I have included the following in my nuspec file:

<?xml version="1.0"?>
<package>
    <metadata minClientVersion="3.3">
        <contentFiles>
            <file src="*.css" target="_css" />
        </contentFiles>
    </metadata>
</package>

However, I get the following error:

MSBUILD : OctoPack error OCTONUGET: The element 'metadata' in namespace 'http://schemas.microsoft.com/packaging/2010/07/nuspec.xsd' has invalid child element 'contentFiles' in namespace 'http://schemas.microsoft.com/packaging/2010/07/nuspec.xsd'. List of possible elements expected: 'language, licenseUrl, projectUrl, requireLicenseAcceptance, summary, tags, frameworkAssemblies, title, references, copyright, authors, description, version, iconUrl, owners, dependencies, id, developmentDependency, releaseNotes' in namespace 'http://schemas.microsoft.com/packaging/2010/07/nuspec.xsd'.

When I look here: https://blog.nuget.org/20160126/nuget-contentFiles-demystified.html, what I've done seems correct. What am I missing?