diff --git a/plugin/type_inet/mysql-test/type_inet/type_inet6_mix_json.result b/plugin/type_inet/mysql-test/type_inet/type_inet6_mix_json.result
index eb35a9a09ca..2b6603a51c3 100644
--- a/plugin/type_inet/mysql-test/type_inet/type_inet6_mix_json.result
+++ b/plugin/type_inet/mysql-test/type_inet/type_inet6_mix_json.result
@@ -40,3 +40,24 @@ ERROR HY000: Illegal parameter data types inet6 and longblob/json for operation
 SELECT a+c FROM t1;
 ERROR HY000: Illegal parameter data types inet6 and longblob for operation '+'
 DROP TABLE t1;
+#
+# MDEV-27668 Assertion `item->type_handler()->is_traditional_scalar_type() || item->type_handler() == type_handler()' failed in Field_inet6::can_optimize_keypart_ref
+#
+CREATE TABLE t1 (i INET6 PRIMARY KEY);
+CREATE TABLE t2 (a VARCHAR(40) CHECK (JSON_VALID(a)));
+SELECT * FROM t1 JOIN t2 ON (i = a);
+i	a
+INSERT INTO t1 VALUES ('::'),('ffff::ffff');
+INSERT INTO t2 VALUES ('{}'),('[]');
+SELECT * FROM t1 JOIN t2 ON (i = a);
+i	a
+DROP TABLE t1, t2;
+CREATE TABLE t1 (i INET6 PRIMARY KEY);
+SELECT * FROM t1 WHERE i<JSON_OBJECT('c','b');
+i
+INSERT INTO t1 VALUES ('::'),('ffff::ffff');
+SELECT * FROM t1 WHERE i<JSON_OBJECT('c','b');
+i
+Warnings:
+Warning	1292	Incorrect inet6 value: '{"c": "b"}'
+DROP TABLE t1;
diff --git a/plugin/type_inet/mysql-test/type_inet/type_inet6_mix_json.test b/plugin/type_inet/mysql-test/type_inet/type_inet6_mix_json.test
index a7aac1ecf00..74a91ff9a05 100644
--- a/plugin/type_inet/mysql-test/type_inet/type_inet6_mix_json.test
+++ b/plugin/type_inet/mysql-test/type_inet/type_inet6_mix_json.test
@@ -48,3 +48,21 @@ SELECT a+b FROM t1;
 SELECT a+c FROM t1;
 
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-27668 Assertion `item->type_handler()->is_traditional_scalar_type() || item->type_handler() == type_handler()' failed in Field_inet6::can_optimize_keypart_ref
+--echo #
+
+CREATE TABLE t1 (i INET6 PRIMARY KEY);
+CREATE TABLE t2 (a VARCHAR(40) CHECK (JSON_VALID(a)));
+SELECT * FROM t1 JOIN t2 ON (i = a);
+INSERT INTO t1 VALUES ('::'),('ffff::ffff');
+INSERT INTO t2 VALUES ('{}'),('[]');
+SELECT * FROM t1 JOIN t2 ON (i = a);
+DROP TABLE t1, t2;
+
+CREATE TABLE t1 (i INET6 PRIMARY KEY);
+SELECT * FROM t1 WHERE i<JSON_OBJECT('c','b');
+INSERT INTO t1 VALUES ('::'),('ffff::ffff');
+SELECT * FROM t1 WHERE i<JSON_OBJECT('c','b');
+DROP TABLE t1;
diff --git a/plugin/type_inet/sql_type_inet.cc b/plugin/type_inet/sql_type_inet.cc
index 9832a84a1b9..26be505ac65 100644
--- a/plugin/type_inet/sql_type_inet.cc
+++ b/plugin/type_inet/sql_type_inet.cc
@@ -936,8 +936,11 @@ public:
       Mixing of two different non-traditional types is currently prevented.
       This may change in the future. For example, INET4 and INET6
       data types can be made comparable.
+      But we allow mixing INET6 to a data type directly inherited from
+      a traditional type, e.g. INET6=VARCHAR/JSON.
     */
-    DBUG_ASSERT(item->type_handler()->is_traditional_scalar_type() ||
+    DBUG_ASSERT(item->type_handler()->type_handler_base_or_self()->
+                  is_traditional_scalar_type() ||
                 item->type_handler() == type_handler());
     return true;
   }
@@ -951,7 +954,8 @@ public:
                           bool is_eq_func) const override
   {
     // See the DBUG_ASSERT comment in can_optimize_keypart_ref()
-    DBUG_ASSERT(item->type_handler()->is_traditional_scalar_type() ||
+    DBUG_ASSERT(item->type_handler()->type_handler_base_or_self()->
+                  is_traditional_scalar_type() ||
                 item->type_handler() == type_handler());
     return true;
   }
diff --git a/sql/sql_type.h b/sql/sql_type.h
index 8ea5b9ddde1..a2e21d81894 100644
--- a/sql/sql_type.h
+++ b/sql/sql_type.h
@@ -3773,6 +3773,11 @@ public:
   {
     return NULL;
   }
+  const Type_handler *type_handler_base_or_self() const
+  {
+    const Type_handler *res= type_handler_base();
+    return res ? res : this;
+  }
   virtual const Type_handler *type_handler_for_comparison() const= 0;
   virtual const Type_handler *type_handler_for_native_format() const
   {