diff --git a/expression/builtin_cast.go b/expression/builtin_cast.go
index d10e3bb829ff0..6ee9d49ed9123 100644
--- a/expression/builtin_cast.go
+++ b/expression/builtin_cast.go
@@ -1848,6 +1848,11 @@ func BuildCastCollationFunction(ctx sessionctx.Context, expr Expression, ec *Exp
 
 // BuildCastFunction builds a CAST ScalarFunction from the Expression.
 func BuildCastFunction(ctx sessionctx.Context, expr Expression, tp *types.FieldType) (res Expression) {
+	argType := expr.GetType()
+	// If source argument's nullable, then target type should be nullable
+	if !mysql.HasNotNullFlag(argType.Flag) {
+		tp.Flag &= ^mysql.NotNullFlag
+	}
 	expr = TryPushCastIntoControlFunctionForHybridType(ctx, expr, tp)
 	var fc functionClass
 	switch tp.EvalType() {
diff --git a/planner/core/logical_plan_test.go b/planner/core/logical_plan_test.go
index 0136545eff430..aec01a73e8442 100644
--- a/planner/core/logical_plan_test.go
+++ b/planner/core/logical_plan_test.go
@@ -110,6 +110,24 @@ func (s *testPlanSuite) TestPredicatePushDown(c *C) {
 	}
 }
 
+// Issue: 31399
+func (s *testPlanSuite) TestImplicitCastNotNullFlag(c *C) {
+	defer testleak.AfterTest(c)()
+	ctx := context.Background()
+	ca := "select count(*) from t3 group by a having bit_and(b) > 1;"
+	comment := Commentf("for %s", ca)
+	stmt, err := s.ParseOneStmt(ca, "", "")
+	c.Assert(err, IsNil, comment)
+	p, _, err := BuildLogicalPlanForTest(ctx, s.ctx, stmt, s.is)
+	c.Assert(err, IsNil)
+	p, err = logicalOptimize(context.TODO(), flagPredicatePushDown|flagJoinReOrder|flagPrunColumns|flagEliminateProjection, p.(LogicalPlan))
+	c.Assert(err, IsNil)
+	// AggFuncs[0] is count; AggFuncs[1] is bit_and, args[0] is return type of the implicit cast
+	castNotNullFlag := (p.(*LogicalProjection).children[0].(*LogicalSelection).children[0].(*LogicalAggregation).AggFuncs[1].Args[0].GetType().Flag) & mysql.NotNullFlag
+	var nullableFlag uint = 0
+	c.Assert(castNotNullFlag, Equals, nullableFlag)
+}
+
 func (s *testPlanSuite) TestEliminateProjectionUnderUnion(c *C) {
 	defer testleak.AfterTest(c)()
 	ctx := context.Background()