diff --git a/app/models/derived_variables/sales_log_variables.rb b/app/models/derived_variables/sales_log_variables.rb
index 3a3e0f9cf..cad8ab15d 100644
--- a/app/models/derived_variables/sales_log_variables.rb
+++ b/app/models/derived_variables/sales_log_variables.rb
@@ -75,7 +75,7 @@ module DerivedVariables::SalesLogVariables
     self.nationality_all = nationality_all_group if nationality_uk_or_prefers_not_to_say?
     self.nationality_all_buyer2 = nationality_all_buyer2_group if nationality2_uk_or_prefers_not_to_say?
 
-    self.numstair = 1 if firststair == 1
+    self.numstair = nil if firststair == 1
     self.mrent = 0 if stairowned == 100
 
     set_encoded_derived_values!(DEPENDENCIES)