List of all users with custom field = '123'

Hi,

I have a custom field VOICETYPE set up for users, and I want a list of all users with VOICETYPE = soprano.

This is the code I have:
'{% set args = { meta_key:'voicetype', meta_value:'soprano', posts_per_page: -1, orderby: 'title', order: 'asc' } %}
{% set users = mb.get_users( args ) %}
{% for user in users %}
{{ user.user_display_name }}
{% endfor %}'

It returns nothing.

How to get what I need?