diff --git a/lib/sequel_postgresql_triggers.rb b/lib/sequel_postgresql_triggers.rb
index 39a67e0..da4e751 100644
--- a/lib/sequel_postgresql_triggers.rb
+++ b/lib/sequel_postgresql_triggers.rb
@@ -78,7 +78,7 @@ def pgt_created_at(table, column, opts={})
 
       # Makes all given columns in the given table immutable, so an exception
       # is raised if there is an attempt to modify the value when updating the
-      # record. Arguments:
+      # record. Allows updating from/to NULL. Arguments:
       # * table : name of table
       # * columns : All columns in the table that should be immutable.  Can end with a hash of options, see module documentation.
       def pgt_immutable(table, *columns)
@@ -89,7 +89,7 @@ def pgt_immutable(table, *columns)
           old = "OLD.#{quote_identifier(c)}"
           new = "NEW.#{quote_identifier(c)}"
           <<-END
-            IF #{new} IS DISTINCT FROM #{old} THEN
+            IF #{new} IS DISTINCT FROM #{old} AND #{old} IS NOT NULL AND #{new} IS NOT NULL THEN
                 RAISE EXCEPTION 'Attempted #{c} update: Old: %, New: %', #{old}, #{new};
             END IF;
           END
@@ -142,7 +142,7 @@ def pgt_sum_cache(main_table, main_table_id_column, sum_column, summed_table, su
       # summed table for the matching id. The join table must have NOT NULL constraints
       # on the foreign keys to the main table and summed table and a
       # composite unique constraint on both foreign keys.
-      # 
+      #
       # Arguments:
       # * opts : option hash, see module documentation, and below.
       # * :main_table: name of table holding sum cache column
diff --git a/spec/sequel_postgresql_triggers_spec.rb b/spec/sequel_postgresql_triggers_spec.rb
index eac5f58..a2794c1 100644
--- a/spec/sequel_postgresql_triggers_spec.rb
+++ b/spec/sequel_postgresql_triggers_spec.rb
@@ -49,31 +49,31 @@
 
       DB[:entries] << {:id=>2, :account_id=>1}
       DB[:accounts].order(:id).select_map(:num_entries).must_equal [2, 0]
-      
+
       DB[:entries] << {:id=>3, :account_id=>nil}
       DB[:accounts].order(:id).select_map(:num_entries).must_equal [2, 0]
-      
+
       DB[:entries].where(:id=>3).update(:account_id=>2)
       DB[:accounts].order(:id).select_map(:num_entries).must_equal [2, 1]
-      
+
       DB[:entries].where(:id=>2).update(:account_id=>2)
       DB[:accounts].order(:id).select_map(:num_entries).must_equal [1, 2]
-      
+
       DB[:entries].where(:id=>2).update(:account_id=>nil)
       DB[:accounts].order(:id).select_map(:num_entries).must_equal [1, 1]
-      
+
       DB[:entries].where(:id=>2).update(:id=>4)
       DB[:accounts].order(:id).select_map(:num_entries).must_equal [1, 1]
-      
+
       DB[:entries].where(:id=>4).update(:account_id=>2)
       DB[:accounts].order(:id).select_map(:num_entries).must_equal [1, 2]
-      
+
       DB[:entries].where(:id=>4).update(:account_id=>nil)
       DB[:accounts].order(:id).select_map(:num_entries).must_equal [1, 1]
-      
+
       DB[:entries].filter(:id=>4).delete
       DB[:accounts].order(:id).select_map(:num_entries).must_equal [1, 1]
-      
+
       DB[:entries].delete
       DB[:accounts].order(:id).select_map(:num_entries).must_equal [0, 0]
     end
@@ -128,11 +128,13 @@
     end
 
     it "Should handle NULL values correctly" do
-      proc{DB[:accounts].update(:balance=>nil)}.must_raise(Sequel::DatabaseError)
+      DB[:accounts].update(:balance=>nil)
       DB[:accounts].delete
       DB[:accounts] << {:id=>1, :balance=>nil}
       DB[:accounts].update(:balance=>nil)
-      proc{DB[:accounts].update(:balance=>0)}.must_raise(Sequel::DatabaseError)
+      DB[:accounts].update(:balance=>0)
+      proc{DB[:accounts].update(:balance=>1)}.must_raise(Sequel::DatabaseError)
+      DB[:accounts].update(:balance=>nil)
     end
   end
 
@@ -157,34 +159,34 @@
 
       DB[:entries] << {:id=>2, :account_id=>1, :amount=>200}
       DB[:accounts].order(:id).select_map(:balance).must_equal [300, 0]
-      
+
       DB[:entries] << {:id=>3, :account_id=>nil, :amount=>500}
       DB[:accounts].order(:id).select_map(:balance).must_equal [300, 0]
-      
+
       DB[:entries].where(:id=>3).update(:account_id=>2)
       DB[:accounts].order(:id).select_map(:balance).must_equal [300, 500]
-      
+
       DB[:entries].exclude(:id=>2).update(:amount=>Sequel.*(:amount, 2))
       DB[:accounts].order(:id).select_map(:balance).must_equal [400, 1000]
-      
+
       DB[:entries].where(:id=>2).update(:account_id=>2)
       DB[:accounts].order(:id).select_map(:balance).must_equal [200, 1200]
-      
+
       DB[:entries].where(:id=>2).update(:account_id=>nil)
       DB[:accounts].order(:id).select_map(:balance).must_equal [200, 1000]
-      
+
       DB[:entries].where(:id=>2).update(:id=>4)
       DB[:accounts].order(:id).select_map(:balance).must_equal [200, 1000]
-      
+
       DB[:entries].where(:id=>4).update(:account_id=>2)
       DB[:accounts].order(:id).select_map(:balance).must_equal [200, 1200]
-      
+
       DB[:entries].where(:id=>4).update(:account_id=>nil)
       DB[:accounts].order(:id).select_map(:balance).must_equal [200, 1000]
-      
+
       DB[:entries].filter(:id=>4).delete
       DB[:accounts].order(:id).select_map(:balance).must_equal [200, 1000]
-      
+
       DB[:entries].delete
       DB[:accounts].order(:id).select_map(:balance).must_equal [0, 0]
     end