diff --git a/expression/integration_test.go b/expression/integration_test.go
index 528b63d73bc58..1d27e687aaf9b 100644
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -238,6 +238,15 @@ func (s *testIntegrationSuite) TestConvertToBit(c *C) {
 	tk.MustExec(`insert t1 value ('09-01-01')`)
 	tk.MustExec(`insert t select a from t1`)
 	tk.MustQuery("select a+0 from t").Check(testkit.Rows("20090101000000"))
+
+	// For issue 20118
+	tk.MustExec("drop table if exists t;")
+	tk.MustExec("create table t(a tinyint, b bit(63));")
+	tk.MustExec("insert ignore  into t values(599999999, -1);")
+	tk.MustQuery("show warnings;").Check(testkit.Rows(
+		"Warning 1690 constant 599999999 overflows tinyint",
+		"Warning 1406 Data Too Long, field len 63"))
+	tk.MustQuery("select * from t;").Check(testkit.Rows("127 \u007f\xff\xff\xff\xff\xff\xff\xff"))
 }
 
 func (s *testIntegrationSuite) TestMathBuiltin(c *C) {
diff --git a/types/datum.go b/types/datum.go
index 341f20eaeef0f..17ae9cfbe8cfe 100644
--- a/types/datum.go
+++ b/types/datum.go
@@ -1287,9 +1287,14 @@ func (d *Datum) convertToMysqlBit(sc *stmtctx.StatementContext, target *FieldTyp
 		uintDatum, err1 := d.convertToUint(sc, target)
 		uintValue, err = uintDatum.GetUint64(), err1
 	}
-	if target.Flen < 64 && uintValue >= 1<<(uint64(target.Flen)) {
+	// Avoid byte size panic, never goto this branch.
+	if target.Flen <= 0 || target.Flen >= 128 {
 		return Datum{}, errors.Trace(ErrDataTooLong.GenWithStack("Data Too Long, field len %d", target.Flen))
 	}
+	if target.Flen < 64 && uintValue >= 1<<(uint64(target.Flen)) {
+		uintValue &= (1 << (uint64(target.Flen))) - 1
+		err = ErrDataTooLong.GenWithStack("Data Too Long, field len %d", target.Flen)
+	}
 	byteSize := (target.Flen + 7) >> 3
 	ret.SetMysqlBit(NewBinaryLiteralFromUint(uintValue, byteSize))
 	return ret, errors.Trace(err)