diff --git a/mysql-test/r/func_gconcat.result b/mysql-test/r/func_gconcat.result
index 6989b89833b..71419b5b2c3 100644
--- a/mysql-test/r/func_gconcat.result
+++ b/mysql-test/r/func_gconcat.result
@@ -728,3 +728,13 @@ f2	group_concat(f1)
 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa	1
 bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb	2
 drop table t1;
+CREATE TABLE t1(a TEXT, b CHAR(20));
+INSERT INTO t1 VALUES ("one.1","one.1"),("two.2","two.2"),("one.3","one.3");
+SELECT GROUP_CONCAT(DISTINCT UCASE(a)) FROM t1;
+GROUP_CONCAT(DISTINCT UCASE(a))
+ONE.1,TWO.2,ONE.3
+SELECT GROUP_CONCAT(DISTINCT UCASE(b)) FROM t1;
+GROUP_CONCAT(DISTINCT UCASE(b))
+ONE.1,TWO.2,ONE.3
+DROP TABLE t1;
+End of 5.0 tests
diff --git a/mysql-test/t/func_gconcat.test b/mysql-test/t/func_gconcat.test
index 3ff4b35873b..0dd82864520 100644
--- a/mysql-test/t/func_gconcat.test
+++ b/mysql-test/t/func_gconcat.test
@@ -497,4 +497,14 @@ select f2,group_concat(f1) from t1 group by f2;
 --disable_metadata
 drop table t1;
 
-# End of 4.1 tests
+#
+# Bug #26815: Unexpected built-in function behavior: group_concat(distinct
+# substring_index())
+# 
+CREATE TABLE t1(a TEXT, b CHAR(20));
+INSERT INTO t1 VALUES ("one.1","one.1"),("two.2","two.2"),("one.3","one.3");
+SELECT GROUP_CONCAT(DISTINCT UCASE(a)) FROM t1;
+SELECT GROUP_CONCAT(DISTINCT UCASE(b)) FROM t1;
+DROP TABLE t1;
+
+--echo End of 5.0 tests
diff --git a/sql/item_sum.cc b/sql/item_sum.cc
index 368dc9a7d38..9d626cb7733 100644
--- a/sql/item_sum.cc
+++ b/sql/item_sum.cc
@@ -417,8 +417,7 @@ Field *Item_sum::create_tmp_field(bool group, TABLE *table,
       2-byte lenght. 
     */
     if (max_length/collation.collation->mbmaxlen > 255 && 
-        max_length/collation.collation->mbmaxlen < UINT_MAX16 &&
-        convert_blob_length)
+        convert_blob_length < UINT_MAX16 && convert_blob_length)
       return new Field_varstring(convert_blob_length, maybe_null,
                                  name, table,
                                  collation.collation);
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index bb57764700d..7e9b1fec12e 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -8805,8 +8805,7 @@ static Field *create_tmp_field_from_item(THD *thd, Item *item, TABLE *table,
       2-byte lenght. 
     */
     else if (item->max_length/item->collation.collation->mbmaxlen > 255 &&
-             item->max_length/item->collation.collation->mbmaxlen < UINT_MAX16
-             && convert_blob_length)
+             convert_blob_length < UINT_MAX16 && convert_blob_length)
       new_field= new Field_varstring(convert_blob_length, maybe_null,
                                      item->name, table,
                                      item->collation.collation);