diff --git a/myisammrg/myrg_write.c b/myisammrg/myrg_write.c
index 0f191edc23c..532709e361d 100644
--- a/myisammrg/myrg_write.c
+++ b/myisammrg/myrg_write.c
@@ -22,9 +22,9 @@ int myrg_write(register MYRG_INFO *info, byte *rec)
 {
   /* [phi] MERGE_WRITE_DISABLED is handled by the else case */
   if (info->merge_insert_method == MERGE_INSERT_TO_FIRST)
-    return mi_write(info->open_tables[0].table,rec);
+    return mi_write((info->current_table=info->open_tables)->table,rec);
   else if (info->merge_insert_method == MERGE_INSERT_TO_LAST)
-    return mi_write(info->end_table[-1].table,rec);
+    return mi_write((info->current_table=info->end_table-1)->table,rec);
   else /* unsupported insertion method */
     return (my_errno= HA_ERR_WRONG_COMMAND);
 }
diff --git a/mysql-test/r/merge.result b/mysql-test/r/merge.result
index 63e0a228aa5..83f8230f11c 100644
--- a/mysql-test/r/merge.result
+++ b/mysql-test/r/merge.result
@@ -544,6 +544,24 @@ insert into t1 values (99,NULL);
 select * from t4 where a+0 > 90;
 a	b
 99	1
+insert t5 values (1,1);
+ERROR 23000: Duplicate entry '1-1' for key 1
+insert t6 values (2,1);
+ERROR 23000: Duplicate entry '2-1' for key 1
+insert t5 values (1,1) on duplicate key update b=b+10;
+insert t6 values (2,1) on duplicate key update b=b+20;
+select * from t5 where a < 3;
+a	b
+1	2
+1	3
+1	4
+1	5
+1	11
+2	2
+2	3
+2	4
+2	5
+2	21
 drop table t6, t5, t4, t3, t2, t1;
 CREATE TABLE t1 (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  PRIMARY KEY  (a,b)) ENGINE=MyISAM;
 INSERT INTO t1 VALUES (1,1), (2,1);
diff --git a/mysql-test/t/merge.test b/mysql-test/t/merge.test
index 1f61e200613..cf55c26fb69 100644
--- a/mysql-test/t/merge.test
+++ b/mysql-test/t/merge.test
@@ -190,6 +190,14 @@ select * from t5 order by a,b;
 select * from t6 order by a,b;
 insert into t1 values (99,NULL);
 select * from t4 where a+0 > 90;
+# bug#4008 - cannot determine a unique key that caused "dupl. key error"
+--error 1062
+insert t5 values (1,1);
+--error 1062
+insert t6 values (2,1);
+insert t5 values (1,1) on duplicate key update b=b+10;
+insert t6 values (2,1) on duplicate key update b=b+20;
+select * from t5 where a < 3;
 drop table t6, t5, t4, t3, t2, t1;
 
 CREATE TABLE t1 (  a int(11) NOT NULL default '0',  b int(11) NOT NULL default '0',  PRIMARY KEY  (a,b)) ENGINE=MyISAM;