diff --git a/ddl/ddl_api.go b/ddl/ddl_api.go
index edf4630e03..77fa5897e8 100644
--- a/ddl/ddl_api.go
+++ b/ddl/ddl_api.go
@@ -760,15 +760,16 @@ func setSetDefaultValue(v types.Datum, col *table.Column) (string, error) {
 		return str, nil
 	}
 
+	ctor := collate.GetCollator(col.Collate)
 	valMap := make(map[string]struct{}, len(col.Elems))
-	dVals := strings.Split(strings.ToLower(str), ",")
+	dVals := strings.Split(str, ",")
 	for _, dv := range dVals {
-		valMap[dv] = struct{}{}
+		valMap[string(ctor.Key(dv))] = struct{}{}
 	}
 	var existCnt int
 	for dv := range valMap {
 		for i := range col.Elems {
-			e := strings.ToLower(col.Elems[i])
+			e := string(ctor.Key(col.Elems[i]))
 			if e == dv {
 				existCnt++
 				break
diff --git a/expression/integration_test.go b/expression/integration_test.go
index 19842a6553..97835c17a9 100755
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -6619,3 +6619,15 @@ func (s *testIntegrationSuite) TestIssue17287(c *C) {
 	tk.MustQuery("execute stmt7 using @val1;").Check(testkit.Rows("1589873945"))
 	tk.MustQuery("execute stmt7 using @val2;").Check(testkit.Rows("1589873946"))
 }
+
+func (s *testIntegrationSerialSuite) TestIssue17891(c *C) {
+	collate.SetNewCollationEnabledForTest(true)
+	defer collate.SetNewCollationEnabledForTest(false)
+
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(id int, value set ('a','b','c') charset utf8mb4 collate utf8mb4_bin default 'a,b ');")
+	tk.MustExec("drop table t")
+	tk.MustExec("create table test(id int, value set ('a','b','c') charset utf8mb4 collate utf8mb4_general_ci default 'a,B ,C');")
+}