diff --git a/include/myisammrg.h b/include/myisammrg.h
index 1d7efbe74d6..b3bca218a44 100644
--- a/include/myisammrg.h
+++ b/include/myisammrg.h
@@ -71,6 +71,7 @@ typedef struct st_myrg_info
   ulong  cache_size;
   uint	 merge_insert_method;
   uint	 tables,options,reclength,keys;
+  uint   key_parts;
   my_bool cache_in_use;
   /* If MERGE children attached to parent. See top comment in ha_myisammrg.cc */
   my_bool children_attached;
diff --git a/mysql-test/main/merge.result b/mysql-test/main/merge.result
index 3761ca2c150..6722dd38b9c 100644
--- a/mysql-test/main/merge.result
+++ b/mysql-test/main/merge.result
@@ -3919,3 +3919,15 @@ ERROR HY000: Unable to open underlying table which is differently defined or of
 DROP TRIGGER trg1;
 DROP TABLE t1;
 DROP TABLE m1;
+#
+# MDEV-31083 ASAN use-after-poison in myrg_attach_children
+#
+CREATE TABLE t1 (f TEXT, FULLTEXT (f)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES ('foo'),('bar');
+CREATE TABLE mrg (f TEXT) ENGINE=MERGE, UNION(t1);
+SELECT * FROM mrg;
+f
+foo
+bar
+DROP TABLE mrg, t1;
+End of 10.5 tests
diff --git a/mysql-test/main/merge.test b/mysql-test/main/merge.test
index 888b41b24bd..ba2ce29ade2 100644
--- a/mysql-test/main/merge.test
+++ b/mysql-test/main/merge.test
@@ -2919,3 +2919,15 @@ set global default_storage_engine=@save_default_storage_engine;
 # Check that all connections opened by test cases in this file are really
 # gone so execution of other tests won't be affected by their presence.
 --source include/wait_until_count_sessions.inc
+
+--echo #
+--echo # MDEV-31083 ASAN use-after-poison in myrg_attach_children
+--echo #
+
+CREATE TABLE t1 (f TEXT, FULLTEXT (f)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES ('foo'),('bar');
+CREATE TABLE mrg (f TEXT) ENGINE=MERGE, UNION(t1);
+SELECT * FROM mrg;
+DROP TABLE mrg, t1;
+
+--echo End of 10.5 tests
diff --git a/storage/myisam/mi_open.c b/storage/myisam/mi_open.c
index 3db424ea997..f47ae48ad0d 100644
--- a/storage/myisam/mi_open.c
+++ b/storage/myisam/mi_open.c
@@ -518,6 +518,7 @@ MI_INFO *mi_open(const char *name, int mode, uint open_flags)
     share->kfile=kfile;
     share->this_process=(ulong) getpid();
     share->last_process= share->state.process;
+    share->base.base_key_parts= base_key_parts;
     share->base.key_parts=key_parts;
     share->base.all_key_parts=key_parts+unique_key_parts;
     if (!(share->last_version=share->state.version))
diff --git a/storage/myisam/myisamdef.h b/storage/myisam/myisamdef.h
index c90d989c975..f84ad6fa184 100644
--- a/storage/myisam/myisamdef.h
+++ b/storage/myisam/myisamdef.h
@@ -132,7 +132,7 @@ typedef struct st_mi_base_info
   uint extra_alloc_bytes;
   uint extra_alloc_procent;
   /* The following are from the header */
-  uint key_parts, all_key_parts;
+  uint key_parts, all_key_parts, base_key_parts;
 } MI_BASE_INFO;
 
 
diff --git a/storage/myisammrg/myrg_open.c b/storage/myisammrg/myrg_open.c
index d9ea4b754f2..4a983684394 100644
--- a/storage/myisammrg/myrg_open.c
+++ b/storage/myisammrg/myrg_open.c
@@ -432,17 +432,20 @@ int myrg_attach_children(MYRG_INFO *m_info, int handle_locking,
       first_child= FALSE;
       m_info->reclength= myisam->s->base.reclength;
       min_keys=  myisam->s->base.keys;
-      key_parts= myisam->s->base.key_parts;
+      key_parts= myisam->s->base.base_key_parts;
       if (*need_compat_check && m_info->rec_per_key_part)
       {
         my_free(m_info->rec_per_key_part);
         m_info->rec_per_key_part= NULL;
       }
-      if (!m_info->rec_per_key_part)
+      if (!m_info->rec_per_key_part || m_info->key_parts != key_parts)
       {
-        if(!(m_info->rec_per_key_part= (ulong*)
-             my_malloc(rg_key_memory_MYRG_INFO,
-                       key_parts * sizeof(long), MYF(MY_WME))))
+        m_info->key_parts= key_parts;
+        /* The +1 is because by my_realloc() don't allow zero length */
+        if (!(m_info->rec_per_key_part= (ulong*)
+              my_realloc(rg_key_memory_MYRG_INFO, m_info->rec_per_key_part,
+                         key_parts * sizeof(long) +1,
+                         MYF(MY_WME | MY_ALLOW_ZERO_PTR | MY_FREE_ON_ERROR))))
           goto err; /* purecov: inspected */
         errpos= 1;
       }
@@ -457,7 +460,8 @@ int myrg_attach_children(MYRG_INFO *m_info, int handle_locking,
     myisam->open_flag|= HA_OPEN_MERGE_TABLE;
 
     /* Check table definition match. */
-    if (m_info->reclength != myisam->s->base.reclength)
+    if (m_info->reclength != myisam->s->base.reclength ||
+        key_parts != myisam->s->base.base_key_parts)
     {
       DBUG_PRINT("error", ("definition mismatch table: '%s'  repair: %d",
                            myisam->filename,