diff --git a/app/models/absolute_identifier.rb b/app/models/absolute_identifier.rb
index c2ab56f..a0c59fb 100644
--- a/app/models/absolute_identifier.rb
+++ b/app/models/absolute_identifier.rb
@@ -82,7 +82,7 @@ def cache_holding_id
   private
 
   def highest_identifier
-    self.class.where(prefix: prefix, pool_identifier: pool_identifier).order(suffix: :desc).pick(:suffix) || last_legacy_identifier
+    self.class.where(prefix: prefix, pool_identifier: pool_identifier).where.not(suffix: nil).order(suffix: :desc).pick(:suffix) || last_legacy_identifier
   end
 
   # The old database ended identifiers for each pool at certain numbers - ensure
diff --git a/app/services/barcode_service.rb b/app/services/barcode_service.rb
index 64e19ad..6e0d46c 100644
--- a/app/services/barcode_service.rb
+++ b/app/services/barcode_service.rb
@@ -1,4 +1,9 @@
 # frozen_string_literal: true
+
+##
+# Given a barcode, be able to get the next one in the sequence. This is coded
+# against the barcodes used specifically at Princeton. Uses Luhn algorithm (*not*
+# codabar algorithm).
 class BarcodeService
   def self.valid?(barcode)
     new(barcode).valid?
diff --git a/spec/models/absolute_identifier_spec.rb b/spec/models/absolute_identifier_spec.rb
index b66ee7e..cb33b4d 100644
--- a/spec/models/absolute_identifier_spec.rb
+++ b/spec/models/absolute_identifier_spec.rb
@@ -65,6 +65,13 @@
         expect(firestone1.full_identifier).to be_blank
       end
     end
+    context "generate_abid==true AFTER generate_abid==false" do
+      it "does not raise an error" do
+        FactoryBot.create(:batch, first_barcode: "32101113344913").absolute_identifiers.first
+        FactoryBot.create(:batch, generate_abid: false).absolute_identifiers.first
+        expect { FactoryBot.create(:batch, first_barcode: "32101113344921").absolute_identifiers.first }.not_to raise_error
+      end
+    end
   end
 
   describe "#synchronize" do