diff --git a/mysql-test/r/myisam.result b/mysql-test/r/myisam.result
index c969b237293..85af643387e 100644
--- a/mysql-test/r/myisam.result
+++ b/mysql-test/r/myisam.result
@@ -2522,6 +2522,17 @@ test.t1	check	error	Size of indexfile is: 1024        Should be: 2048
 test.t1	check	warning	Size of datafile is: 14       Should be: 7
 test.t1	check	error	Corrupt
 DROP TABLE t1;
+#
+# MDEV-3870 - Valgrind warnings on OPTIMIZE MyISAM or Aria TABLE with
+#             disabled keys
+#
+CREATE TABLE t1 (a INT, KEY(a)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (4),(3),(1),(0);
+ALTER TABLE t1 DISABLE KEYS;
+OPTIMIZE TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	optimize	status	OK
+DROP TABLE t1;
 show variables like 'myisam_block_size';
 Variable_name	Value
 myisam_block_size	1024
diff --git a/mysql-test/suite/maria/optimize.result b/mysql-test/suite/maria/optimize.result
index 9cce55d6199..a78e8e469aa 100644
--- a/mysql-test/suite/maria/optimize.result
+++ b/mysql-test/suite/maria/optimize.result
@@ -6,3 +6,14 @@ OPTIMIZE TABLE t1;
 Table	Op	Msg_type	Msg_text
 test.t1	optimize	status	OK
 drop table t1;
+#
+# MDEV-3870 - Valgrind warnings on OPTIMIZE MyISAM or Aria TABLE with
+#             disabled keys
+#
+CREATE TABLE t1 (a INT, KEY(a)) ENGINE=Aria;
+INSERT INTO t1 VALUES (4),(3),(1),(0);
+ALTER TABLE t1 DISABLE KEYS;
+OPTIMIZE TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	optimize	status	OK
+DROP TABLE t1;
diff --git a/mysql-test/suite/maria/optimize.test b/mysql-test/suite/maria/optimize.test
index 6b310b1d1a6..b1fc250cd29 100644
--- a/mysql-test/suite/maria/optimize.test
+++ b/mysql-test/suite/maria/optimize.test
@@ -160,3 +160,13 @@ INSERT /*! IGNORE */ INTO t1 VALUES  ('urxjxqvwabikpugvexxbxdpxjkeqiuhhuadbcuhoz
 check table t1;
 OPTIMIZE TABLE t1;
 drop table t1;
+
+--echo #
+--echo # MDEV-3870 - Valgrind warnings on OPTIMIZE MyISAM or Aria TABLE with
+--echo #             disabled keys
+--echo #
+CREATE TABLE t1 (a INT, KEY(a)) ENGINE=Aria;
+INSERT INTO t1 VALUES (4),(3),(1),(0);
+ALTER TABLE t1 DISABLE KEYS;
+OPTIMIZE TABLE t1;
+DROP TABLE t1;
diff --git a/mysql-test/t/myisam.test b/mysql-test/t/myisam.test
index 8323890b028..43c12b42bc9 100644
--- a/mysql-test/t/myisam.test
+++ b/mysql-test/t/myisam.test
@@ -1749,6 +1749,16 @@ CHECK TABLE t1;
 
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-3870 - Valgrind warnings on OPTIMIZE MyISAM or Aria TABLE with
+--echo #             disabled keys
+--echo #
+CREATE TABLE t1 (a INT, KEY(a)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (4),(3),(1),(0);
+ALTER TABLE t1 DISABLE KEYS;
+OPTIMIZE TABLE t1;
+DROP TABLE t1;
+
 #
 # Check some variables
 #
diff --git a/storage/maria/ma_check.c b/storage/maria/ma_check.c
index 3be4465c71e..ecdc512ef69 100644
--- a/storage/maria/ma_check.c
+++ b/storage/maria/ma_check.c
@@ -3119,10 +3119,8 @@ int maria_sort_index(HA_CHECK *param, register MARIA_HA *info, char *name)
   for (key= 0,keyinfo= &share->keyinfo[0]; key < share->base.keys ;
        key++,keyinfo++)
   {
-    if (! maria_is_key_active(share->state.key_map, key))
-      continue;
-
-    if (share->state.key_root[key] != HA_OFFSET_ERROR)
+    if (maria_is_key_active(share->state.key_map, key) &&
+        share->state.key_root[key] != HA_OFFSET_ERROR)
     {
       index_pos[key]=param->new_file_pos;	/* Write first block here */
       if (sort_one_index(param,info,keyinfo,share->state.key_root[key],
diff --git a/storage/myisam/mi_check.c b/storage/myisam/mi_check.c
index 73558ce77b3..ff9ea4b82cb 100644
--- a/storage/myisam/mi_check.c
+++ b/storage/myisam/mi_check.c
@@ -1944,10 +1944,8 @@ int mi_sort_index(HA_CHECK *param, register MI_INFO *info, char * name)
   for (key= 0,keyinfo= &share->keyinfo[0]; key < share->base.keys ;
        key++,keyinfo++)
   {
-    if (! mi_is_key_active(info->s->state.key_map, key))
-      continue;
-
-    if (share->state.key_root[key] != HA_OFFSET_ERROR)
+    if (mi_is_key_active(info->s->state.key_map, key) &&
+        share->state.key_root[key] != HA_OFFSET_ERROR)
     {
       index_pos[key]=param->new_file_pos;	/* Write first block here */
       if (sort_one_index(param,info,keyinfo,share->state.key_root[key],