When I launch a heat template with constraints custom_constraint and allowed_values on one parameter it fails

We have just upgraded from Juno to Liberty.

A heat template (that used to work in Juno) declares the following parameter:
  instance_type:
    type: string
    description: The flavour the webserver is to run on
    default: m2.xsmall
    constraints:
      - allowed_values: [m2.xsmall, m2.small, m1.small]
        description:
          Must be a valid flavour, limited to the smaller ones available
      - custom_constraint: nova.flavor

When I try to load it via the dashboard, the drop down for the parameter no longer shows the allowed values, instead now listing all of the possible values. And when I select a value and launch the stack, I get the error message:
Error: ERROR: Parameter 'instance_type' is invalid: Must be a valid NeCTAR flavour, limited to the smaller ones available
And the stack fails to launch.

However, it launches quite happily from the command line.

If I comment either of the constraints out, the template works in the dashboard.

If I comment out the allowed_values constraint, I get a list of the flavors that I can select. So I'm guessing that you have written some helpful code to list the available flavors if I constrain it to nova.flavors: and that this is clashing with the allowed_values somehow...