diff --git a/app/models/log.rb b/app/models/log.rb
index e15869634..486ffa252 100644
--- a/app/models/log.rb
+++ b/app/models/log.rb
@@ -119,12 +119,13 @@ class Log < ApplicationRecord
 
   def blank_invalid_non_setup_fields!
     setup_ids = form.setup_sections.flat_map(&:subsections).flat_map(&:questions).map(&:id)
+    fields_to_keep = setup_ids + %w[hhmemb]
 
     2.times do
       next if valid?
 
       errors.each do |error|
-        next if setup_ids.include?(error.attribute.to_s)
+        next if fields_to_keep.include?(error.attribute.to_s)
 
         question = form.questions.find { |q| q.id == error.attribute.to_s }
         if question&.type == "checkbox"
diff --git a/spec/models/log_spec.rb b/spec/models/log_spec.rb
index 3494f152f..b35737c67 100644
--- a/spec/models/log_spec.rb
+++ b/spec/models/log_spec.rb
@@ -76,5 +76,16 @@ RSpec.describe Log, type: :model do
         expect(model.previous_la_known).to be_nil
       end
     end
+
+    context "when hhmemb is invalid for a lettings log" do
+      subject(:model) { build(:lettings_log, :setup_completed, hhmemb: 1, joint: 1) }
+
+      it "does not blank it" do
+        model.valid?
+        model.blank_invalid_non_setup_fields!
+        expect(model.hhmemb).to be(1)
+        expect(model.joint).to be_nil
+      end
+    end
   end
 end