Rails 5 fixture returning nil

Overall Description of Problem:

My fixture for my WeightClass model is returning nil during testing. However, I know the fixtures are valid because when I run rails db:fixtures:load RAILS_ENV=test, all of my fixtures are successfully loaded into my test database.

On a side note, if anybody could point me to the Rails source code that deals with how the weight_classes method (and other methods that turn fixtures into model classes) works in a test, it would be greatly appreciated.

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Relevant Files

I have a model for weight classes as seen below (app/models/weight_class.rb):

class WeightClass < ApplicationRecord
  validates :name,        presence: true, length: { maximum: 15 }
  validates :name_abbr,   presence: true, length: { maximum: 3  }
  validates :gender,      presence: true, length: { maximum: 1  }
  validates :lower_bound, numericality: { greater_than: 0 }, allow_nil: true
  validates :upper_bound, numericality: { greater_than: 0 }, allow_nil: true

  validates_with LowerOrUpperBoundRequiredValidator
  validates_with RangeValidator, if: :lower_and_upper_bound_present?

  private
    def lower_and_upper_bound_present?
      self.lower_bound and self.upper_bound
    end
end

And it's migration here:

class CreateWeightClasses < ActiveRecord::Migration[5.0]
  def change
    create_table :weight_classes do |t|
      t.string :name, limit: 15, null: false
      t.string :name_abbr, limit: 3, null: false
      t.string :gender, limit: 1, null: false
      t.decimal :lower_bound, precision: 5, scale: 2
      t.decimal :upper_bound, precision: 5, scale: 2

      t.timestamps
    end
  end
end

The fixture in question here (test/fixtures/weight_classes.yml):

middleweight_men:
  name: Middleweight
  name_abbr: MW
  gender: m
  lower_bound: 73
  upper_bound: 87

The problem is, when I call this fixture in a test to see if it's valid:

require 'test_helper'

class WeightClassTest < ActiveSupport::TestCase
  def setup
    @middlewight_men = weight_classes(:middleweight_men)
  end

  test "should be valid" do
    assert @middleweight_men.valid?
  end
end

I get this error:

NoMethodError: undefined method `valid?' for nil:NilClass