What am I doing wrong here - or does PowerShell v5 not support separate class files and inheritance?

foo.ps1:

class foo
{
   [STRING] $test = 'Hello World';

   [STRING] PrintTest()
   {
      return $this.test;
   }
}

bar.ps1:

Import-Module "$PSScriptRoot\foo.ps1"

class bar : foo
{ }

CONSOLE

PS C:\Script> Import-Module ./bar.ps1;

PS C:\Script> $myBar = New-Object bar;

PS C:\Script> $myBar.PrintTest();

Method invocation failed because [bar] does not contain a method named 'PrintTest'.

If it put the foo and bar classes in the same file it works fine. However the classes I'm building are rather large and I want to separate them.