Declarative syntax: ${env.WORKSPACE} not set in environment block

I got the following in my jenkins file:

pipeline {
agent any

environment {
SOURCE = "${env.WORKSPACE}
mydirectory"
// The above is not working? WORKSPACE is null as you can see later.
HTDOCS = "C:\\Apache24\\htdocs
"
}

stages {
stage('Prepare') {
steps {
echo "WORKSPACE ${env.WORKSPACE}" // correct
echo "SOURCE ${env.SOURCE}" // wrong, is set to null\myDirectoy
echo "INEW_HTDOCS ${env.INEW_HTDOCS}" // correct
}
}

....