diff --git a/types/field_type.go b/types/field_type.go
index ae4ca8bc1d614..767d297f634ff 100644
--- a/types/field_type.go
+++ b/types/field_type.go
@@ -72,6 +72,7 @@ func AggFieldType(tps []*FieldType) *FieldType {
 		}
 		mtp := MergeFieldType(currType.Tp, t.Tp)
 		currType.Tp = mtp
+		currType.Flag = mergeTypeFlag(currType.Flag, t.Flag)
 	}
 
 	return &currType
@@ -308,6 +309,13 @@ func MergeFieldType(a byte, b byte) byte {
 	return fieldTypeMergeRules[ia][ib]
 }
 
+// mergeTypeFlag merges two MySQL type flag to a new one
+// currently only NotNullFlag is checked
+// todo more flag need to be checked, for example: UnsignedFlag
+func mergeTypeFlag(a, b uint) uint {
+	return a & (b&mysql.NotNullFlag | ^mysql.NotNullFlag)
+}
+
 func getFieldTypeIndex(tp byte) int {
 	itp := int(tp)
 	if itp < fieldTypeTearFrom {
diff --git a/types/field_type_test.go b/types/field_type_test.go
index 31c5fd97a2e4d..9b50d65e9a7d9 100644
--- a/types/field_type_test.go
+++ b/types/field_type_test.go
@@ -300,6 +300,32 @@ func (s *testFieldTypeSuite) TestAggFieldType(c *C) {
 		}
 	}
 }
+func (s *testFieldTypeSuite) TestAggFieldTypeForTypeFlag(c *C) {
+	types := []*FieldType{
+		NewFieldType(mysql.TypeLonglong),
+		NewFieldType(mysql.TypeLonglong),
+	}
+
+	aggTp := AggFieldType(types)
+	c.Assert(aggTp.Tp, Equals, mysql.TypeLonglong)
+	c.Assert(aggTp.Flag, Equals, uint(0))
+
+	types[0].Flag = mysql.NotNullFlag
+	aggTp = AggFieldType(types)
+	c.Assert(aggTp.Tp, Equals, mysql.TypeLonglong)
+	c.Assert(aggTp.Flag, Equals, uint(0))
+
+	types[0].Flag = 0
+	types[1].Flag = mysql.NotNullFlag
+	aggTp = AggFieldType(types)
+	c.Assert(aggTp.Tp, Equals, mysql.TypeLonglong)
+	c.Assert(aggTp.Flag, Equals, uint(0))
+
+	types[0].Flag = mysql.NotNullFlag
+	aggTp = AggFieldType(types)
+	c.Assert(aggTp.Tp, Equals, mysql.TypeLonglong)
+	c.Assert(aggTp.Flag, Equals, mysql.NotNullFlag)
+}
 
 func (s *testFieldTypeSuite) TestAggregateEvalType(c *C) {
 	defer testleak.AfterTest(c)()