Ruby on Rails validation on input form generating errors

My Tracks model has an integer column called 'bpm'

When I try to submit the form to create a new track and add it to the database; the following error occurs:

Field can't be blank

This occurs despite the field being filled in and the validations surrounding it have been carried out, perhaps there's an error in the post request?

The post-request functions correctly when I do not validate the bpm field.

views > tracks > _form.html.haml

.columns
  .column.is-8.is-centered
    = simple_form_for @track, html: { multipart: true } do |f|
      = f.error_notification
      .columns
        .field.column.is-9
          .control
            = f.input :name , required: true, input_html: { class: "input"}, wrapper: false, label_html: { class:"label" }
        .field.column
          .control
            = f.input :price, required: true, input_html: { class:"input", maxlength: 7  }, wrapper: false, label_html: { class:"label" }
        .field.column
          .control
            = f.input :bpm, required: true, input_html: {class:"input", maxlength: 3}, wrapper: false, label_html: { class:"label" }
      .field
        .control
          = f.input :description, required: true, input_html: { class:"textarea" }, wrapper: false, label_html: { class:"label" }
      .columns
        .field.column.is-4
          .control
            %label.label Genre
            .control.has-icons-left
              %span.select
                = f.input_field :genre, collection: Track::GENRE, prompt: "Select type"
              %span.icon.is-small.is-left
                %i.fa.fa-tag
      .field
        .control
          %label.label Add images
          .file
            %label.file-label
              = f.input :image, as: :file, input_html: { class:"file-input track-image" }, label: false, wrapper: false
              %span.file-cta
                %span.file-icon
                  %i.fa.fa-upload
                %span.file-label Choose a file…
      %output#list
      %hr/
      .field.is-grouped
        .control
          = f.button :submit, class: 'button is-warning'
          = link_to 'Cancel', tracks_path, class:'button is-light'

models > track.rb

class Track < ApplicationRecord
  before_destroy :not_referenced_by_any_line_item
  belongs_to :user, optional: true
  has_many :line_items

  mount_uploader :image, ImageUploader
  serialize :image, JSON #sqlite

  validates :name, :genre, :price, :bpm, presence: true
  validates :description, length: { maximum: 1000, too_long: "Up to %{count} characters allowed"}, presence: true
  validates :bpm, length: { maximum: 3 }
  validates :price, length: { maximum: 5 }
  GENRE = %w{ Trap Hip-Hop R&B Funk Electro-R&B }


  private
  def not_referenced_by_any_line_item
    unless line_items.empty?
      errors.add(:base, "Line items present")
      throw :abort
    end
  end

end

track schema:

create_table "tracks", force: :cascade do |t|
    t.string "name"
    t.text "description"
    t.decimal "price", precision: 5, scale: 2, default: "0.0"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.string "image"
    t.integer "user_id"
    t.string "genre"
    t.integer "bpm"
  end