Plain textboxes with kartik\slider\Slider. What i did wrong?

Question

<?php
use kartik\slider\Slider;
echo $form->field($model, 'input_field')->widget(Slider::className(), [
        'pluginOptions' => [
            // 'pluginConflict' => true,
            'min' => 0,
            'max' => 50,
            'step' => 10,
            'tooltip' => 'always',
        ]
    ]);

I am using this widget: http://demos.krajee.com/slider but just getting plain textboxes. What am I doing wrong? I was unable to find a solution in the internet.