diff --git a/mysql-test/main/vector2.result b/mysql-test/main/vector2.result
index 99bff8aa4d67f..1d39de5213ae4 100644
--- a/mysql-test/main/vector2.result
+++ b/mysql-test/main/vector2.result
@@ -565,4 +565,33 @@ tmp	CREATE TABLE `tmp` (
 c
 drop table tmp, t1, t2;
 set sql_mode=@old_sql_mode;
+#
+# MDEV-40486 ER_TOO_BIG_FIELDLENGTH or assertion failure upon creating vector from blob
+#
+## Original testcase
+CREATE TABLE t (a TEXT) AS SELECT '[1]' AS a;
+CREATE TABLE tt AS SELECT VEC_FROMTEXT(a) AS f FROM t;
+ERROR 42000: Column length too big for column 'f' (max = 16383); use BLOB or TEXT instead
+DROP TABLE t;
+## Conversion from max varchar length
+CREATE TABLE t (a VARCHAR(16383)) AS
+SELECT concat('[1', repeat(',1', 8190), ']') AS a;
+CREATE TABLE tt AS SELECT VEC_FROMTEXT(a) AS f FROM t;
+DROP TABLE t,tt;
+## Conversion to max vector dimension
+CREATE TABLE tt AS
+SELECT VEC_FROMTEXT(concat('[1', repeat(',1', 16382), ']')) AS f;
+DROP TABLE tt;
+CREATE TABLE tt AS
+SELECT VEC_FROMTEXT(concat('[1', repeat(',1', 16383), ']')) AS f;
+ERROR 42000: Column length too big for column 'f' (max = 16383); use BLOB or TEXT instead
+## Zero length: no underflow
+## Note that CREATE ... SELECT raises warning as error
+SELECT VEC_FROMTEXT('') AS f;
+f
+NULL
+Warnings:
+Warning	4037	Unexpected end of JSON text in argument 1 to function 'VEC_FromText'
+CREATE TABLE tt AS SELECT VEC_FROMTEXT('') AS f;
+ERROR HY000: Unexpected end of JSON text in argument 1 to function 'VEC_FromText'
 # End of 11.8 tests
diff --git a/mysql-test/main/vector2.test b/mysql-test/main/vector2.test
index be2493011aacd..c31322584a287 100644
--- a/mysql-test/main/vector2.test
+++ b/mysql-test/main/vector2.test
@@ -447,4 +447,35 @@ drop table tmp, t1, t2;
 
 set sql_mode=@old_sql_mode;
 
+--echo #
+--echo # MDEV-40486 ER_TOO_BIG_FIELDLENGTH or assertion failure upon creating vector from blob
+--echo #
+
+--echo ## Original testcase
+CREATE TABLE t (a TEXT) AS SELECT '[1]' AS a;
+--error ER_TOO_BIG_FIELDLENGTH
+CREATE TABLE tt AS SELECT VEC_FROMTEXT(a) AS f FROM t;
+DROP TABLE t;
+
+--echo ## Conversion from max varchar length
+CREATE TABLE t (a VARCHAR(16383)) AS
+  SELECT concat('[1', repeat(',1', 8190), ']') AS a;
+CREATE TABLE tt AS SELECT VEC_FROMTEXT(a) AS f FROM t;
+DROP TABLE t,tt;
+
+--echo ## Conversion to max vector dimension
+CREATE TABLE tt AS
+  SELECT VEC_FROMTEXT(concat('[1', repeat(',1', 16382), ']')) AS f;
+DROP TABLE tt;
+
+--error ER_TOO_BIG_FIELDLENGTH
+CREATE TABLE tt AS
+  SELECT VEC_FROMTEXT(concat('[1', repeat(',1', 16383), ']')) AS f;
+
+--echo ## Zero length: no underflow
+--echo ## Note that CREATE ... SELECT raises warning as error
+SELECT VEC_FROMTEXT('') AS f;
+--error ER_JSON_EOS
+CREATE TABLE tt AS SELECT VEC_FROMTEXT('') AS f;
+
 --echo # End of 11.8 tests
diff --git a/sql/item_vectorfunc.cc b/sql/item_vectorfunc.cc
index 13e458d7675d6..e5138f85bdafd 100644
--- a/sql/item_vectorfunc.cc
+++ b/sql/item_vectorfunc.cc
@@ -181,12 +181,13 @@ Item_func_vec_fromtext::Item_func_vec_fromtext(THD *thd, Item *a)
 
 bool Item_func_vec_fromtext::fix_length_and_dec(THD *thd)
 {
+  uint maxlen= args[0]->max_char_length();
   decimals= 0;
   /* Worst case scenario, for a valid input we have a string of the form:
      [1,2,3,4,5,...] single digit numbers.
-     This means we can have (max_length - 1) / 2 floats.
-     Each float takes 4 bytes, so we do (max_length - 1) * 2. */
-  fix_length_and_charset((args[0]->max_length - 1) * 2, &my_charset_bin);
+     This means we can have (maxlen - 1) / 2 floats.
+     Each float takes 4 bytes, so we do (maxlen - 1) * 2. */
+  fix_length_and_charset(maxlen ? (maxlen - 1) * 2 : 0, &my_charset_bin);
   set_maybe_null();
   return false;
 }
diff --git a/sql/item_vectorfunc.h b/sql/item_vectorfunc.h
index babf634704108..0631958fa0565 100644
--- a/sql/item_vectorfunc.h
+++ b/sql/item_vectorfunc.h
@@ -98,7 +98,15 @@ class Item_func_vec_fromtext: public Item_str_func
   const Type_handler *type_handler() const override
   { return &type_handler_vector; }
   Field *create_field_for_create_select(MEM_ROOT *root, TABLE *table) override
-  { return create_table_field_from_handler(root, table); }
+  {
+    if (max_length > MAX_FIELD_VARCHARLENGTH)
+    {
+      my_error(ER_TOO_BIG_FIELDLENGTH, MYF(0), name.str,
+               static_cast<ulong>(MAX_FIELD_VARCHARLENGTH / sizeof(float)));
+      return NULL;
+    }
+    return create_table_field_from_handler(root, table);
+  }
   LEX_CSTRING func_name_cstring() const override
   {
     static LEX_CSTRING name= { STRING_WITH_LEN("VEC_FromText") };