Future parser interpolation with [] after variable prefixed with underscore

When a variable containing a hash has a name prefixed with an underscore:

$_foo = {

  'a' => 'b',

}

notify { "${_foo['a']}": }

and run `puppet apply --parser=future` you get the following error:

Error: Evaluation Error: The value 'a' cannot be converted to Numeric. on node uagent2.corp.puppetlabs.net

Error: Evaluation Error: The value 'a' cannot be converted to Numeric. on node uagent2.corp.puppetlabs.net

That looks like it might be assuming that the hash is an array?

The following applies without error:

$foo = {

  'a' => 'b',

}

notify { "${foo['a']}": }

This fails with the future parser on puppet 3.7.5, and also fails against master (at 7ba7ae8f3b221896a3b77beb6778b7a895671c7d)