Bad assignation of partials with includePath: true

[andresbravog]

Having gruntfile

    haml:
      compile:
        files:
          "assets/compiled/templates.js": [ "assets/javascripts/**/*.hamlc" ]
        options:
          target: 'js'
          language: 'coffee'
          includePath: true
          pathRelativeTo: './assets/javascripts/backbone/',
          dependencies:
            $: 'jquery',
            _: 'underscore'
            I18n: 'i18n'

and some partials in my views I end up with the error

Running "haml:compile" (haml) task
>> [stdin]:2:41: error: the variable "_basket_prize" can't be assigned with ?= because it has not been declared before
>> window.HAML.apps/lottery/show/templates/_basket_prize ?= {}
>>                                         ^^^^^^^^^^^^^
Warning: Haml failed to compile. Use --force to continue.

as you can see views are correctly assigned with

window.HAML['tasks'] = function(context) {
    return (function() {
...

but templates are wrongly assigned with

window.HAML.apps/lottery/_basket_prize['jst'] = function(context) {
    return (function() {
      var $o;
      $o = [];
      $o.push("<div class='name'></div>\n<div class='quantity'></div>\n<div class='remove'>&times;</div>");
      return $o.join("\n").replace(/\s(?:id|class)=(['"])(\1)/mg, "");
    }).call(context);
  };

Any idea how to solve this issue?