diff --git a/app/models/request_type.rb b/app/models/request_type.rb
index 1a5b2c21af..0f4f59d3c3 100644
--- a/app/models/request_type.rb
+++ b/app/models/request_type.rb
@@ -82,7 +82,7 @@ class RequestTypePlatePurpose < ApplicationRecord
   # eg. library_creation has a lower order than sequencing
   validates :order, presence: true
   validates :order, numericality: { integer_only: true }
-  validates :morphology, numericality: { in: 0..2 } # 0..2 is the range of morphologies as defined in ln36-41
+  validates :morphology, numericality: { in: 0...MORPHOLOGIES.length }
   validates :request_class, presence: true, inclusion: { in: ->(_) { [Request, *Request.descendants] } }
 
   serialize :request_parameters