diff --git a/heap/hp_write.c b/heap/hp_write.c
index a60d32eecb6..bc94e3bfae4 100644
--- a/heap/hp_write.c
+++ b/heap/hp_write.c
@@ -105,7 +105,7 @@ int hp_rb_write_key(HP_INFO *info, HP_KEYDEF *keyinfo, const byte *record,
   custom_arg.key_length= hp_rb_make_key(keyinfo, info->recbuf, record, recpos);
   if (keyinfo->flag & HA_NOSAME)
   {
-    custom_arg.search_flag= SEARCH_FIND | SEARCH_SAME | SEARCH_UPDATE;
+    custom_arg.search_flag= SEARCH_FIND | SEARCH_UPDATE;
     keyinfo->rb_tree.flag= TREE_NO_DUPS;
   }
   else
diff --git a/mysql-test/r/heap_btree.result b/mysql-test/r/heap_btree.result
index b63eaf7e48c..4b05e8f44e1 100644
--- a/mysql-test/r/heap_btree.result
+++ b/mysql-test/r/heap_btree.result
@@ -256,3 +256,6 @@ SELECT INDEX_LENGTH FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA=DATABASE()
 INDEX_LENGTH
 21
 DROP TABLE t1;
+CREATE TABLE t1 (a INT, UNIQUE USING BTREE(a)) ENGINE=MEMORY;
+INSERT INTO t1 VALUES(NULL),(NULL);
+DROP TABLE t1;
diff --git a/mysql-test/t/heap_btree.test b/mysql-test/t/heap_btree.test
index f1b9d290885..fb715fccefe 100644
--- a/mysql-test/t/heap_btree.test
+++ b/mysql-test/t/heap_btree.test
@@ -176,4 +176,12 @@ UPDATE t1 SET val=1;
 SELECT INDEX_LENGTH FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA=DATABASE() AND TABLE_NAME='t1';
 DROP TABLE t1;
 
+#
+# BUG#12873 - BTREE index on MEMORY table with multiple NULL values doesn't
+# work properly
+#
+CREATE TABLE t1 (a INT, UNIQUE USING BTREE(a)) ENGINE=MEMORY;
+INSERT INTO t1 VALUES(NULL),(NULL);
+DROP TABLE t1;
+
 # End of 4.1 tests
diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index 32a86b0f39a..476d5536581 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -3140,7 +3140,7 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
   uint db_create_options, used_fields;
   enum db_type old_db_type,new_db_type;
   bool need_copy_table;
-  bool no_table_reopen= FALSE;
+  bool no_table_reopen= FALSE, varchar= FALSE;
   DBUG_ENTER("mysql_alter_table");
 
   thd->proc_info="init";
@@ -3344,6 +3344,8 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
   Field **f_ptr,*field;
   for (f_ptr=table->field ; (field= *f_ptr) ; f_ptr++)
   {
+    if (field->type() == MYSQL_TYPE_STRING)
+      varchar= TRUE;
     /* Check if field should be dropped */
     Alter_drop *drop;
     drop_it.rewind();
@@ -3605,12 +3607,18 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
     better have a negative test here, instead of positive, like
     alter_info->flags & ALTER_ADD_COLUMN|ALTER_ADD_INDEX|...
     so that ALTER TABLE won't break when somebody will add new flag
+
+    MySQL uses frm version to determine the type of the data fields and
+    their layout. See Field_string::type() for details.
+    Thus, if the table is too old we may have to rebuild the data to
+    update the layout.
   */
   need_copy_table= (alter_info->flags &
                     ~(ALTER_CHANGE_COLUMN_DEFAULT|ALTER_OPTIONS) ||
                     (create_info->used_fields &
                      ~(HA_CREATE_USED_COMMENT|HA_CREATE_USED_PASSWORD)) ||
-                    table->s->tmp_table);
+                    table->s->tmp_table ||
+                    (table->s->frm_version < FRM_VER_TRUE_VARCHAR && varchar));
   create_info->frm_only= !need_copy_table;
 
   /*