diff --git a/mysql-test/r/merge.result b/mysql-test/r/merge.result
index 79d8f019ce3..6e14e9a56a8 100644
--- a/mysql-test/r/merge.result
+++ b/mysql-test/r/merge.result
@@ -681,3 +681,13 @@ t3	1	a	1	a	A	NULL	NULL	NULL	YES	BTREE
 t3	1	a	2	b	A	NULL	NULL	NULL	YES	BTREE	
 t3	1	a	3	c	A	NULL	NULL	NULL	YES	BTREE	
 drop table t1, t2, t3;
+CREATE TABLE t1 ( a INT AUTO_INCREMENT PRIMARY KEY, b VARCHAR(10), UNIQUE (b) )
+ENGINE=MyISAM;
+CREATE TABLE t2 ( a INT AUTO_INCREMENT, b VARCHAR(10), INDEX (a), INDEX (b) )
+ENGINE=MERGE UNION (t1) INSERT_METHOD=FIRST;
+INSERT INTO t2 (b) VALUES (1) ON DUPLICATE KEY UPDATE b=2;
+INSERT INTO t2 (b) VALUES (1) ON DUPLICATE KEY UPDATE b=3;
+SELECT b FROM t2;
+b
+3
+DROP TABLE t1, t2;
diff --git a/mysql-test/t/merge.test b/mysql-test/t/merge.test
index 508f9da225e..999dd2bed9b 100644
--- a/mysql-test/t/merge.test
+++ b/mysql-test/t/merge.test
@@ -306,3 +306,15 @@ show index from t3;
 
 drop table t1, t2, t3;
 
+#
+# Bug#10400 - Improperly-defined MERGE table crashes with INSERT ... ON DUPLICATE KEY UPDATE
+#
+CREATE TABLE t1 ( a INT AUTO_INCREMENT PRIMARY KEY, b VARCHAR(10), UNIQUE (b) )
+  ENGINE=MyISAM;
+CREATE TABLE t2 ( a INT AUTO_INCREMENT, b VARCHAR(10), INDEX (a), INDEX (b) )
+  ENGINE=MERGE UNION (t1) INSERT_METHOD=FIRST;
+INSERT INTO t2 (b) VALUES (1) ON DUPLICATE KEY UPDATE b=2;
+INSERT INTO t2 (b) VALUES (1) ON DUPLICATE KEY UPDATE b=3;
+SELECT b FROM t2;
+DROP TABLE t1, t2;
+
diff --git a/sql/ha_myisammrg.h b/sql/ha_myisammrg.h
index 3bc9c11d4be..7348096b695 100644
--- a/sql/ha_myisammrg.h
+++ b/sql/ha_myisammrg.h
@@ -37,7 +37,7 @@ class ha_myisammrg: public handler
   {
     return (HA_REC_NOT_IN_SEQ | HA_AUTO_PART_KEY | HA_READ_RND_SAME |
 	    HA_NULL_IN_KEY | HA_CAN_INDEX_BLOBS | HA_FILE_BASED |
-            HA_CAN_INSERT_DELAYED);
+            HA_CAN_INSERT_DELAYED | HA_ANY_INDEX_MAY_BE_UNIQUE);
   }
   ulong index_flags(uint inx, uint part, bool all_parts) const
   {
diff --git a/sql/handler.h b/sql/handler.h
index de932cd51a4..75e83082d10 100644
--- a/sql/handler.h
+++ b/sql/handler.h
@@ -73,6 +73,7 @@
 #define HA_HAS_CHECKSUM        (1 << 24)
 /* Table data are stored in separate files (for lower_case_table_names) */
 #define HA_FILE_BASED	       (1 << 26)
+#define HA_ANY_INDEX_MAY_BE_UNIQUE (1 << 30)
 
 
 /* bits in index_flags(index_number) for what you can do with index */
diff --git a/sql/sql_insert.cc b/sql/sql_insert.cc
index a3a42ce385d..0258f2c3e07 100644
--- a/sql/sql_insert.cc
+++ b/sql/sql_insert.cc
@@ -682,7 +682,7 @@ int write_record(TABLE *table,COPY_INFO *info)
 
 err:
   if (key)
-    my_afree(key);
+    my_safe_afree(key,table->max_unique_length,MAX_KEY_LENGTH);
   info->last_errno= error;
   table->file->print_error(error,MYF(0));
   DBUG_RETURN(1);
diff --git a/sql/table.cc b/sql/table.cc
index 012defa116d..d9000933ee0 100644
--- a/sql/table.cc
+++ b/sql/table.cc
@@ -688,7 +688,12 @@ int openfrm(const char *name, const char *alias, uint db_stat, uint prgflag,
       set_if_bigger(outparam->max_key_length,keyinfo->key_length+
 		    keyinfo->key_parts);
       outparam->total_key_length+= keyinfo->key_length;
-      if (keyinfo->flags & HA_NOSAME)
+      /*
+        MERGE tables do not have unique indexes. But every key could be
+        an unique index on the underlying MyISAM table. (Bug #10400)
+      */
+      if ((keyinfo->flags & HA_NOSAME) ||
+          (ha_option & HA_ANY_INDEX_MAY_BE_UNIQUE))
         set_if_bigger(outparam->max_unique_length,keyinfo->key_length);
     }
     if (primary_key < MAX_KEY &&