How do I set my own value as the NameID in SAML2

Question

I am using SimpleSamlPHP I am trying to setup the sp-remote metadata for a service provider, and I want to send him the user's email in the NameID / subject of the assertion.

I keep getting:

Unable to generate NameID. Check the userid.attribute option.
Falling back to transient NameID.

I have no authproc entries in the idp-hosted metadata, and the relevant entries I have in the SP-remote metadata are:

'NameIDFormat' => 'urn:oasis:names:tc:SAML:2.0:nameid-format:emailAddress',
'authproc' => [
        0 => [
            'class' => 'saml:AttributeNameID',
            'attribute' => 'email',
            'Format' => 'urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress',
            'SPNameQualifier' => false,
            'NameQualifier' => false
        ],
],

The email attribute (I see it in the debug output) is:

Array
(
    [username] => Array
        (
            [0] => 42
        )

    [first_name] => Array
        (
            [0] => Itay
        )

    [last_name] => Array
        (
            [0] => Moav
        )

    [email] => Array
        (
            [0] => itay.moav@gumboo.org
        )

)

I am using version 1.17.6