diff --git a/mysql-test/main/func_str.result b/mysql-test/main/func_str.result
index 68cda5fae95..bcc9721d146 100644
--- a/mysql-test/main/func_str.result
+++ b/mysql-test/main/func_str.result
@@ -5355,5 +5355,27 @@ HEX(INSERT(_utf8 0xD18FD18E, 2, 1, 0x20))
 D120D18E
 DROP VIEW v1;
 #
+# MDEV-28686 Assertion `0' in Type_handler_string_result::make_sort_key or unexpected result
+#
+CREATE TABLE t (s DATE, e DATE, PERIOD FOR p(s,e));
+INSERT INTO t (s,e) VALUES ('1970-01-01','1970-01-02'),('1980-01-01','1980-01-02');
+SET sql_mode='';
+SELECT e, GROUP_CONCAT(s) FROM t GROUP BY CONVERT((LPAD(e, -1) AND e) USING utf8);
+e	GROUP_CONCAT(s)
+1970-01-02	1970-01-01,1980-01-01
+DROP TABLE t;
+CREATE TABLE t (s DATE, e DATE, PERIOD FOR p(s,e));
+INSERT INTO t (s,e) VALUES ('1970-01-01','1970-01-02'),('1980-01-01','1980-01-02');
+SET sql_mode='';
+SELECT DISTINCT CONVERT((LPAD(e, -1) AND e) USING utf8) FROM t;
+CONVERT((LPAD(e, -1) AND e) USING utf8)
+NULL
+SET sql_mode=STRICT_TRANS_TABLES;
+SELECT DISTINCT CONVERT((LPAD(e, -1) AND e) USING utf8) FROM t;
+CONVERT((LPAD(e, -1) AND e) USING utf8)
+NULL
+DROP TABLE t;
+SET sql_mode=DEFAULT;
+#
 # End of 10.6 tests
 #
diff --git a/mysql-test/main/func_str.test b/mysql-test/main/func_str.test
index d632250ba52..9735019c132 100644
--- a/mysql-test/main/func_str.test
+++ b/mysql-test/main/func_str.test
@@ -2402,6 +2402,24 @@ CREATE VIEW v1 AS SELECT HEX(INSERT(_utf8 0xD18FD18E, 2, 1, 0x20));
 SELECT * FROM v1;
 DROP VIEW v1;
 
+--echo #
+--echo # MDEV-28686 Assertion `0' in Type_handler_string_result::make_sort_key or unexpected result
+--echo #
+
+CREATE TABLE t (s DATE, e DATE, PERIOD FOR p(s,e));
+INSERT INTO t (s,e) VALUES ('1970-01-01','1970-01-02'),('1980-01-01','1980-01-02');
+SET sql_mode='';
+SELECT e, GROUP_CONCAT(s) FROM t GROUP BY CONVERT((LPAD(e, -1) AND e) USING utf8);
+DROP TABLE t;
+
+CREATE TABLE t (s DATE, e DATE, PERIOD FOR p(s,e));
+INSERT INTO t (s,e) VALUES ('1970-01-01','1970-01-02'),('1980-01-01','1980-01-02');
+SET sql_mode='';
+SELECT DISTINCT CONVERT((LPAD(e, -1) AND e) USING utf8) FROM t;
+SET sql_mode=STRICT_TRANS_TABLES;
+SELECT DISTINCT CONVERT((LPAD(e, -1) AND e) USING utf8) FROM t;
+DROP TABLE t;
+SET sql_mode=DEFAULT;
 
 --echo #
 --echo # End of 10.6 tests
diff --git a/sql/item_strfunc.cc b/sql/item_strfunc.cc
index 72d7520839b..15821ad530a 100644
--- a/sql/item_strfunc.cc
+++ b/sql/item_strfunc.cc
@@ -61,13 +61,17 @@ size_t username_char_length= USERNAME_CHAR_LENGTH;
   Calculate max length of string from length argument to LEFT and RIGHT
 */
 
-static uint32 max_length_for_string(Item *item)
+static uint32 max_length_for_string(Item *item, bool *neg)
 {
+  *neg= false;
   ulonglong length= item->val_int();
   if (item->null_value)
     return 0;
   if (length > (ulonglong) LONGLONG_MAX && !item->unsigned_flag)
+  {
+    *neg= true;
     return 0; // Negative
+  }
   if (length > (ulonglong) INT_MAX32)
   {
     /* Limit string length to maxium string length in MariaDB (2G) */
@@ -1664,7 +1668,8 @@ void Item_str_func::left_right_max_length()
   uint32 char_length= args[0]->max_char_length();
   if (args[1]->can_eval_in_optimize())
   {
-    uint32 length= max_length_for_string(args[1]);
+    bool neg;
+    uint32 length= max_length_for_string(args[1], &neg);
     set_if_smaller(char_length, length);
   }
   fix_char_length(char_length);
@@ -3078,7 +3083,8 @@ bool Item_func_repeat::fix_length_and_dec()
   DBUG_ASSERT(collation.collation != NULL);
   if (args[1]->can_eval_in_optimize())
   {
-    uint32 length= max_length_for_string(args[1]);
+    bool neg;
+    uint32 length= max_length_for_string(args[1], &neg);
     ulonglong char_length= (ulonglong) args[0]->max_char_length() * length;
     fix_char_length_ulonglong(char_length);
     return false;
@@ -3152,7 +3158,8 @@ bool Item_func_space::fix_length_and_dec()
   collation.set(default_charset(), DERIVATION_COERCIBLE, MY_REPERTOIRE_ASCII);
   if (args[0]->can_eval_in_optimize())
   {
-    fix_char_length_ulonglong(max_length_for_string(args[0]));
+    bool neg;
+    fix_char_length_ulonglong(max_length_for_string(args[0], &neg));
     return false;
   }
   max_length= MAX_BLOB_WIDTH;
@@ -3278,7 +3285,10 @@ bool Item_func_pad::fix_length_and_dec()
   DBUG_ASSERT(collation.collation->mbmaxlen > 0);
   if (args[1]->can_eval_in_optimize())
   {
-    fix_char_length_ulonglong(max_length_for_string(args[1]));
+    bool neg;
+    fix_char_length_ulonglong(max_length_for_string(args[1], &neg));
+    if (neg)
+      set_maybe_null();
     return false;
   }
   max_length= MAX_BLOB_WIDTH;