diff --git a/mysql-test/r/key.result b/mysql-test/r/key.result
index b8387389075..28824de94ce 100644
--- a/mysql-test/r/key.result
+++ b/mysql-test/r/key.result
@@ -222,3 +222,16 @@ explain select 1 from t1 where id =2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ref	id	id	4	const	1	Using where; Using index
 drop table t1;
+CREATE TABLE t1 (numeropost mediumint(8) unsigned NOT NULL default '0', numreponse int(10) unsigned NOT NULL auto_increment, PRIMARY KEY (numeropost,numreponse), UNIQUE KEY numreponse (numreponse));
+INSERT INTO t1 (numeropost,numreponse) VALUES ('1','1'),('1','2'),('2','3'),('2','4');
+SELECT numeropost FROM t1 WHERE numreponse='1';
+numeropost
+1
+EXPLAIN SELECT numeropost FROM t1 WHERE numreponse='1';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	const	numreponse	numreponse	4	const	1	Using index
+FLUSH TABLES;
+SELECT numeropost FROM t1 WHERE numreponse='1';
+numeropost
+1
+drop table t1;
diff --git a/mysql-test/t/key.test b/mysql-test/t/key.test
index 4fa85477318..8d399abfec9 100644
--- a/mysql-test/t/key.test
+++ b/mysql-test/t/key.test
@@ -216,3 +216,15 @@ explain select name from t1 where id =2;
 ALTER TABLE t1 DROP PRIMARY KEY, ADD INDEX (id);
 explain select 1 from t1 where id =2;
 drop table t1;
+
+#
+# Test of problem with key read (Bug #3666)
+#
+
+CREATE TABLE t1 (numeropost mediumint(8) unsigned NOT NULL default '0', numreponse int(10) unsigned NOT NULL auto_increment, PRIMARY KEY (numeropost,numreponse), UNIQUE KEY numreponse (numreponse));
+INSERT INTO t1 (numeropost,numreponse) VALUES ('1','1'),('1','2'),('2','3'),('2','4');
+SELECT numeropost FROM t1 WHERE numreponse='1';
+EXPLAIN SELECT numeropost FROM t1 WHERE numreponse='1';
+FLUSH TABLES;
+SELECT numeropost FROM t1 WHERE numreponse='1';
+drop table t1;
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 2cfeb171153..abe239e7d4d 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -3638,7 +3638,7 @@ make_join_readinfo(JOIN *join, uint options)
       table->status=STATUS_NO_RECORD;
       tab->read_first_record= join_read_const;
       tab->read_record.read_record= join_no_more_records;
-      if (table->used_keys.is_set(tab->index) &&
+      if (table->used_keys.is_set(tab->ref.key) &&
           !table->no_keyread)
       {
         table->key_read=1;
@@ -5967,7 +5967,7 @@ join_read_const_table(JOIN_TAB *tab, POSITION *pos)
   }
   else
   {
-    if (!table->key_read && table->used_keys.is_set(tab->index) &&
+    if (!table->key_read && table->used_keys.is_set(tab->ref.key) &&
 	!table->no_keyread)
     {
       table->key_read=1;