Warning on installation with docpad 6.54.6

[churel]

Hello,
Thx you for this awesome plugin.
When I install docpad-plugin-hapi on a docpad 6.54.6 I have this warning :

warning: Failed to load the plugin: docpad-plugin-hapi at /node_modules/docpad-plugin-hapi 
The error follows:

warning: An error occured: 
TypeError: Object function BasePlugin(opts) {
      this.getConfig = __bind(this.getConfig, this);
      this.setConfig = __bind(this.setConfig, this);
      var config, docpad, me;
      me = this;
      docpad = opts.docpad, config = opts.config;
      this.docpad = docpad;
      this.bindListeners();
      this.config = extendr.deepClone(this.config);
      this.instanceConfig = extendr.deepClone(this.instanceConfig);
      this.initialConfig = this.config;
      this.setConfig(config);
      if (this.isEnabled() === false) {
        return this;
      }
      this.addListeners();
      this;
    } has no method 'extend'
    at module.exports

I have no issue on docpad version 6.64.0. I haven't test with other versions of docpad.
The plugin define his dependency to "docpad": ">=6"