Can't quite get these transforms working...

TransformTest1.psm1

# this works, but only if I import with using
class StringTransformAttribute : System.Management.Automation.ArgumentTransformationAttribute
{
    [object] Transform([System.Management.Automation.EngineIntrinsics]$EngineIntrinsics, [object]$InputData)
    {
        return "a different string"
    }
}

function Test-StringTransform
{
    param
    (
        [StringTransform()]
        $String
    )

    $String
}

TransformTest2.psm1

# this works with Import-Module
class StringTransform : System.Management.Automation.ArgumentTransformationAttribute
{
    [object] Transform([System.Management.Automation.EngineIntrinsics]$EngineIntrinsics, [object]$InputData)
    {
        return "a different string"
    }
}

function Test-StringTransform
{
    param
    (
        [StringTransform()]
        $String
    )

    $String
}