diff --git a/myisam/mi_create.c b/myisam/mi_create.c
index f99a2c655d2..da3330627a7 100644
--- a/myisam/mi_create.c
+++ b/myisam/mi_create.c
@@ -533,6 +533,21 @@ int mi_create(const char *name,uint keys,MI_KEYDEF *keydefs,
     create_flag=MY_DELETE_OLD;
   }
 
+  /*
+    If a MRG_MyISAM table is in use, the mapped MyISAM tables are open,
+    but no entry is made in the table cache for them.
+    A TRUNCATE command checks for the table in the cache only and could
+    be fooled to believe, the table is not open.
+    Pull the emergency brake in this situation. (Bug #8306)
+  */
+  if (test_if_reopen(filename))
+  {
+    my_printf_error(0, "MyISAM table '%s' is in use "
+                    "(most likely by a MERGE table). Try FLUSH TABLES.",
+                    MYF(0), name + dirname_length(name));
+    goto err;
+  }
+
   if ((file= my_create_with_symlink(linkname_ptr,
 				    filename,
 				    0, O_RDWR | O_TRUNC,
diff --git a/myisam/mi_open.c b/myisam/mi_open.c
index 2a327e4bd35..2c85a03c6f4 100644
--- a/myisam/mi_open.c
+++ b/myisam/mi_open.c
@@ -50,7 +50,7 @@ if (pos > end_pos)             \
 ** In MySQL the server will handle version issues.
 ******************************************************************************/
 
-static MI_INFO *test_if_reopen(char *filename)
+MI_INFO *test_if_reopen(char *filename)
 {
   LIST *pos;
 
diff --git a/myisam/myisamdef.h b/myisam/myisamdef.h
index 736ce3f3869..916932c9eb3 100644
--- a/myisam/myisamdef.h
+++ b/myisam/myisamdef.h
@@ -705,6 +705,7 @@ void mi_copy_status(void* to,void *from);
 my_bool mi_check_status(void* param);
 void mi_disable_non_unique_index(MI_INFO *info, ha_rows rows);
 
+extern MI_INFO *test_if_reopen(char *filename);
 my_bool check_table_is_closed(const char *name, const char *where);
 int mi_open_datafile(MI_INFO *info, MYISAM_SHARE *share, File file_to_dup);
 int mi_open_keyfile(MYISAM_SHARE *share);
diff --git a/mysql-test/r/myisam.result b/mysql-test/r/myisam.result
index d155a14bb60..eaed7c620e3 100644
--- a/mysql-test/r/myisam.result
+++ b/mysql-test/r/myisam.result
@@ -555,3 +555,21 @@ select count(*) from t1 where a is null;
 count(*)
 2
 drop table t1;
+create table t1 (c1 int, index(c1));
+create table t2 (c1 int, index(c1)) engine=merge union=(t1);
+insert into t1 values (1);
+flush tables;
+select * from t2;
+c1
+1
+flush tables;
+truncate table t1;
+insert into t1 values (1);
+flush tables;
+select * from t2;
+c1
+1
+truncate table t1;
+ERROR HY000: MyISAM table 't1' is in use (most likely by a MERGE table). Try FLUSH TABLES.
+insert into t1 values (1);
+drop table t1,t2;
diff --git a/mysql-test/t/myisam.test b/mysql-test/t/myisam.test
index c8ed7910b76..0babd1f9401 100644
--- a/mysql-test/t/myisam.test
+++ b/mysql-test/t/myisam.test
@@ -524,3 +524,29 @@ explain select count(*) from t1 where a is null;
 select count(*) from t1 where a is null;
 drop table t1;
 
+#
+# Bug #8306: TRUNCATE leads to index corruption 
+#
+create table t1 (c1 int, index(c1));
+create table t2 (c1 int, index(c1)) engine=merge union=(t1);
+insert into t1 values (1);
+# Close all tables.
+flush tables;
+# Open t2 and (implicitly) t1.
+select * from t2;
+# Truncate after flush works (unless another threads reopens t2 in between).
+flush tables;
+truncate table t1;
+insert into t1 values (1);
+# Close all tables.
+flush tables;
+# Open t2 and (implicitly) t1.
+select * from t2;
+# Truncate t1, wich was not recognized as open without the bugfix.
+# Now, it should fail with a table-in-use error message.
+--error 1105
+truncate table t1;
+# The insert used to fail on the crashed table.
+insert into t1 values (1);
+drop table t1,t2;
+