I can't get Flask-Bootstrap to render columns correctly

Question

I'm copy/pasting code from the Bootstrap site and even so, I can't get anything to render.

Instead of:

column______________column________________column

I get:

column
column
column

__init__.py

from flask import Flask
from flask_bootstrap import Bootstrap

app = Flask(__name__)
bootstrap = Bootstrap(app)

from app import routes

base.html

{% extends "bootstrap/base.html" %}

    {% block content %}
        <div class="container">
            {% block app_content %}
            {% endblock %}
        </div>
    {% endblock %}

index.html

{% extends "base.html" %}

{% block app_content %}

    <div class="container-fluid">
      <div class="row">
        <div class="col-md-2">
          One of three columns
        </div>
        <div class="col-md-8">
          One of three columns
        </div>
        <div class="col-md-2">
          One of three columns
        </div>
      </div>
    </div>


{% endblock %}