diff --git a/expression/builtin_control.go b/expression/builtin_control.go
index 13f539648e049..5eaa39c703811 100644
--- a/expression/builtin_control.go
+++ b/expression/builtin_control.go
@@ -189,7 +189,14 @@ func (c *caseWhenFunctionClass) getFunction(ctx sessionctx.Context, args []Expre
 	}
 	fieldTp.Decimal, fieldTp.Flen = decimal, flen
 	if fieldTp.EvalType().IsStringKind() && !isBinaryStr {
-		fieldTp.Charset, fieldTp.Collate = charset.CharsetUTF8MB4, charset.CollationUTF8MB4
+		fieldTp.Charset, fieldTp.Collate = DeriveCollationFromExprs(ctx, args...)
+		if fieldTp.Charset == charset.CharsetBin && fieldTp.Collate == charset.CollationBin {
+			// When args are Json and Numerical type(eg. Int), the fieldTp is String.
+			// Both their charset/collation is binary, but the String need a default charset/collation.
+			fieldTp.Charset, fieldTp.Collate = charset.GetDefaultCharsetAndCollate()
+		}
+	} else {
+		fieldTp.Charset, fieldTp.Collate = charset.CharsetBin, charset.CollationBin
 	}
 	if isBinaryFlag {
 		fieldTp.Flag |= mysql.BinaryFlag
diff --git a/expression/expr_to_pb_test.go b/expression/expr_to_pb_test.go
index a82c0e9276629..027a1dbd360b1 100644
--- a/expression/expr_to_pb_test.go
+++ b/expression/expr_to_pb_test.go
@@ -558,7 +558,7 @@ func (s *testEvaluatorSuite) TestControlFunc2Pb(c *C) {
 	pbExprs, err := ExpressionsToPBList(sc, controlFuncs, client)
 	c.Assert(err, IsNil)
 	jsons := []string{
-		"{\"tp\":10000,\"children\":[{\"tp\":201,\"val\":\"gAAAAAAAAAE=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":63,\"charset\":\"\"},\"has_distinct\":false},{\"tp\":201,\"val\":\"gAAAAAAAAAI=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":63,\"charset\":\"\"},\"has_distinct\":false},{\"tp\":201,\"val\":\"gAAAAAAAAAM=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":63,\"charset\":\"\"},\"has_distinct\":false}],\"sig\":4208,\"field_type\":{\"tp\":3,\"flag\":128,\"flen\":-1,\"decimal\":0,\"collate\":63,\"charset\":\"\"},\"has_distinct\":false}",
+		"{\"tp\":10000,\"children\":[{\"tp\":201,\"val\":\"gAAAAAAAAAE=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":63,\"charset\":\"\"},\"has_distinct\":false},{\"tp\":201,\"val\":\"gAAAAAAAAAI=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":63,\"charset\":\"\"},\"has_distinct\":false},{\"tp\":201,\"val\":\"gAAAAAAAAAM=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":63,\"charset\":\"\"},\"has_distinct\":false}],\"sig\":4208,\"field_type\":{\"tp\":3,\"flag\":128,\"flen\":-1,\"decimal\":0,\"collate\":63,\"charset\":\"binary\"},\"has_distinct\":false}",
 		"{\"tp\":10000,\"children\":[{\"tp\":201,\"val\":\"gAAAAAAAAAE=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":63,\"charset\":\"\"},\"has_distinct\":false},{\"tp\":201,\"val\":\"gAAAAAAAAAI=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":63,\"charset\":\"\"},\"has_distinct\":false},{\"tp\":201,\"val\":\"gAAAAAAAAAM=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":63,\"charset\":\"\"},\"has_distinct\":false}],\"sig\":4107,\"field_type\":{\"tp\":3,\"flag\":128,\"flen\":24,\"decimal\":0,\"collate\":63,\"charset\":\"binary\"},\"has_distinct\":false}",
 		"{\"tp\":10000,\"children\":[{\"tp\":201,\"val\":\"gAAAAAAAAAE=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":63,\"charset\":\"\"},\"has_distinct\":false},{\"tp\":201,\"val\":\"gAAAAAAAAAI=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":63,\"charset\":\"\"},\"has_distinct\":false}],\"sig\":4101,\"field_type\":{\"tp\":3,\"flag\":128,\"flen\":24,\"decimal\":0,\"collate\":63,\"charset\":\"binary\"},\"has_distinct\":false}",
 		"null",
diff --git a/expression/integration_test.go b/expression/integration_test.go
index 56d6b734b3069..a4cf05b46b392 100644
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -9207,6 +9207,18 @@ func (s *testIntegrationSuite) TestRefineArgNullValues(c *C) {
 	))
 }
 
+func (s *testIntegrationSerialSuite) TestIssue26662(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 t1;")
+	tk.MustExec("create table t1(a varchar(36) NOT NULL) ENGINE = InnoDB DEFAULT CHARSET = utf8 COLLATE = utf8_general_ci;")
+	tk.MustExec("set names utf8;")
+	tk.MustQuery("select t2.b from (select t1.a as b from t1 union all select t1.a as b from t1) t2 where case when (t2.b is not null) then t2.b else '' end > '1234567';").
+		Check(testkit.Rows())
+}
+
 func (s *testIntegrationSuite) TestConstPropNullFunctions(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("use test")