diff --git a/mysql-test/r/innodb.result b/mysql-test/r/innodb.result
index f47c78c9768..c7aef8ed792 100644
--- a/mysql-test/r/innodb.result
+++ b/mysql-test/r/innodb.result
@@ -1685,3 +1685,12 @@ explain select * from t1 order by a,b,c,d;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	Using filesort
 drop table t1;
+create table t1 (a char(1), b char(1), key(a, b)) engine=innodb;
+insert into t1 values ('8', '6'), ('4', '7');
+select min(a) from t1;
+min(a)
+4
+select min(b) from t1 where a='8';
+min(b)
+6
+drop table t1;
diff --git a/mysql-test/t/innodb.test b/mysql-test/t/innodb.test
index a14370c6543..b966ea5b281 100644
--- a/mysql-test/t/innodb.test
+++ b/mysql-test/t/innodb.test
@@ -1230,4 +1230,13 @@ select * from t1 order by a,b,c,d;
 explain select * from t1 order by a,b,c,d;
 drop table t1;
 
+#
+# BUG#11039,#13218 Wrong key length in min()
+#
+
+create table t1 (a char(1), b char(1), key(a, b)) engine=innodb;
+insert into t1 values ('8', '6'), ('4', '7');
+select min(a) from t1;
+select min(b) from t1 where a='8';
+drop table t1;
 # End of 4.1 tests
diff --git a/sql/opt_sum.cc b/sql/opt_sum.cc
index 4ab506cc4e1..cb8e3c2d273 100644
--- a/sql/opt_sum.cc
+++ b/sql/opt_sum.cc
@@ -661,7 +661,8 @@ static bool find_key_for_maxmin(bool max_fl, TABLE_REF *ref,
               If key_part2 may be NULL, then we want to find the first row
               that is not null
             */
-            ref->key_buff[ref->key_length++]= 1;
+            ref->key_buff[ref->key_length]= 1;
+            ref->key_length+= part->store_length;
             *range_fl&= ~NO_MIN_RANGE;
             *range_fl|= NEAR_MIN;                // > NULL
           }