Step is not defined

  import_module('%s.%s' % (app_config.name, module_to_search))                                 
  File "/usr/local/pyenv/versions/3.5.0/lib/python3.5/importlib/__init__.py", line 126, in import
_module                                                                                          
    return _bootstrap._gcd_import(name[level:], package, level)                                  
  File "<frozen importlib._bootstrap>", line 986, in _gcd_import                                 
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load                              
  File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked                     
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked                              
  File "<frozen importlib._bootstrap_external>", line 662, in exec_module                        
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed                   
  File "/home/treehouse/workspace/learning_site/courses/admin.py", line 6, in <module>           
    admin.site.register(Step)                                                                    
NameError: name 'Step' is not defined 

from django.db import models

# Create your models here.
class Course(models.Model):
    created_at = models.DateTimeField(auto_now_add=True)
    title = models.CharField(max_length=255)
    description = models.TextField()

    def __str__(self):
        return self.title

class Step(models.Model):
    title = models.CharField(max_length=255)
    description = models.TextField()
    order = models.IntegerField(default=0)
    course = models.ForeignKey(Course)

    def __str__(self):
        return self.title

from django.contrib import admin

from .models import Course

admin.site.register(Course)
admin.site.register(Step)