Field true not found on type Site

I’m using EntityGraphQL with an asp.netcore app that uses Entity Framework

This query works

{
  sites(filter: "id==1") {
    id
    name
    isActive
  }
}

But this one fails with: "Field true not found on type Site"

{
  sites(filter: "isActive==true") {
    id
    name
    isActive
  }
}

The documentation says I should be able to do that, why is it not working?