diff --git a/expression/builtin_control.go b/expression/builtin_control.go
index d327b1a694c9e..7ef46c4890e08 100644
--- a/expression/builtin_control.go
+++ b/expression/builtin_control.go
@@ -142,14 +142,22 @@ func (c *caseWhenFunctionClass) getFunction(ctx sessionctx.Context, args []Expre
 	for i := 1; i < l; i += 2 {
 		fieldTps = append(fieldTps, args[i].GetType())
 		decimal = mathutil.Max(decimal, args[i].GetType().Decimal)
-		flen = mathutil.Max(flen, args[i].GetType().Flen)
+		if args[i].GetType().Flen == -1 {
+			flen = -1
+		} else if flen != -1 {
+			flen = mathutil.Max(flen, args[i].GetType().Flen)
+		}
 		isBinaryStr = isBinaryStr || types.IsBinaryStr(args[i].GetType())
 		isBinaryFlag = isBinaryFlag || !types.IsNonBinaryStr(args[i].GetType())
 	}
 	if l%2 == 1 {
 		fieldTps = append(fieldTps, args[l-1].GetType())
 		decimal = mathutil.Max(decimal, args[l-1].GetType().Decimal)
-		flen = mathutil.Max(flen, args[l-1].GetType().Flen)
+		if args[l-1].GetType().Flen == -1 {
+			flen = -1
+		} else if flen != -1 {
+			flen = mathutil.Max(flen, args[l-1].GetType().Flen)
+		}
 		isBinaryStr = isBinaryStr || types.IsBinaryStr(args[l-1].GetType())
 		isBinaryFlag = isBinaryFlag || !types.IsNonBinaryStr(args[l-1].GetType())
 	}
diff --git a/expression/expr_to_pb_test.go b/expression/expr_to_pb_test.go
index 8ab887f7cfb4d..54dc91ee69688 100644
--- a/expression/expr_to_pb_test.go
+++ b/expression/expr_to_pb_test.go
@@ -443,7 +443,7 @@ func (s *testEvaluatorSuite) TestControlFunc2Pb(c *C) {
 
 	pbExprs := ExpressionsToPBList(sc, controlFuncs, client)
 	jsons := []string{
-		"{\"tp\":10000,\"children\":[{\"tp\":201,\"val\":\"gAAAAAAAAAE=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":46,\"charset\":\"\"}},{\"tp\":201,\"val\":\"gAAAAAAAAAI=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":46,\"charset\":\"\"}},{\"tp\":201,\"val\":\"gAAAAAAAAAM=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":46,\"charset\":\"\"}}],\"sig\":4208,\"field_type\":{\"tp\":3,\"flag\":128,\"flen\":0,\"decimal\":0,\"collate\":46,\"charset\":\"\"}}",
+		"{\"tp\":10000,\"children\":[{\"tp\":201,\"val\":\"gAAAAAAAAAE=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":46,\"charset\":\"\"}},{\"tp\":201,\"val\":\"gAAAAAAAAAI=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":46,\"charset\":\"\"}},{\"tp\":201,\"val\":\"gAAAAAAAAAM=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":46,\"charset\":\"\"}}],\"sig\":4208,\"field_type\":{\"tp\":3,\"flag\":128,\"flen\":-1,\"decimal\":0,\"collate\":46,\"charset\":\"\"}}",
 		"{\"tp\":10000,\"children\":[{\"tp\":201,\"val\":\"gAAAAAAAAAE=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":46,\"charset\":\"\"}},{\"tp\":201,\"val\":\"gAAAAAAAAAI=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":46,\"charset\":\"\"}},{\"tp\":201,\"val\":\"gAAAAAAAAAM=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":46,\"charset\":\"\"}}],\"sig\":4107,\"field_type\":{\"tp\":3,\"flag\":128,\"flen\":-1,\"decimal\":0,\"collate\":63,\"charset\":\"binary\"}}",
 		"{\"tp\":10000,\"children\":[{\"tp\":201,\"val\":\"gAAAAAAAAAE=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":46,\"charset\":\"\"}},{\"tp\":201,\"val\":\"gAAAAAAAAAI=\",\"sig\":0,\"field_type\":{\"tp\":3,\"flag\":0,\"flen\":-1,\"decimal\":-1,\"collate\":46,\"charset\":\"\"}}],\"sig\":4101,\"field_type\":{\"tp\":3,\"flag\":128,\"flen\":-1,\"decimal\":0,\"collate\":63,\"charset\":\"binary\"}}",
 		"null",
diff --git a/expression/integration_test.go b/expression/integration_test.go
index d7065c7d26e13..e97d9e838590c 100644
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -4022,6 +4022,13 @@ func (s *testIntegrationSuite) TestIssues(c *C) {
 	tk.MustExec("create table t(a int)")
 	tk.MustExec("insert t values (1)")
 	tk.MustQuery("select * from t where cast(a as binary)").Check(testkit.Rows("1"))
+
+	// for issue #16351
+	tk.MustExec("drop table if exists t2")
+	tk.MustExec("create table t2(a int, b varchar(20))")
+	tk.MustExec(`insert into t2 values(1,"1111"),(2,"2222"),(3,"3333"),(4,"4444"),(5,"5555"),(6,"6666"),(7,"7777"),(8,"8888"),(9,"9999"),(10,"0000")`)
+	tk.MustQuery(`select (@j := case when substr(t2.b,1,3)=@i then 1 else @j+1 end) from t2, (select @j := 0, @i := "0") tt limit 10`).Check(testkit.Rows(
+		"1", "2", "3", "4", "5", "6", "7", "8", "9", "10"))
 }
 
 func (s *testIntegrationSuite) TestInPredicate4UnsignedInt(c *C) {