TypeError while using sample Model

[X17]

while running program: django 1.7

Model

class Article(models.Model):

title = models.CharField(max_length=255) gallery = GalleryForeignKey('galleries.Gallery', blank=True, null=True, on_delete=models.SET_NULL)

Couldn't reconstruct field gallery on testapp.Article: init() takes at least 2 arguments (1 given)

thanks in advance.