fix json error when struct field is slice and is nil

copier.go

@@ -174,6 +174,10 @@ func copier(toValue interface{}, fromValue interface{}, opt Option) (err error)
 	}
 
 	if from.Kind() == reflect.Slice && to.Kind() == reflect.Slice && fromType.ConvertibleTo(toType) {
+		// if both from and to slices are nil, do nothing
+		if to.IsNil() && from.IsNil() {
+			return
+		}
 		if to.IsNil() {
 			slice := reflect.MakeSlice(reflect.SliceOf(to.Type().Elem()), from.Len(), from.Cap())
 			to.Set(slice)

copier_test.go

@@ -2,6 +2,7 @@ package copier_test
 
 import (
 	"database/sql"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"reflect"
@@ -1611,6 +1612,39 @@ func TestDeepCopySimpleTime(t *testing.T) {
 	}
 }
 
+func TestDeepCopyWithNilByteSlices(t *testing.T) {
+	type From struct {
+		Flags json.RawMessage
+	}
+
+	type To struct {
+		Flags json.RawMessage
+	}
+
+	from := From{}
+	to := To{}
+
+	err := copier.CopyWithOption(&to, &from, copier.Option{
+		DeepCopy: true,
+	})
+	if err != nil {
+		t.Error("error when copying")
+	}
+	if to.Flags != nil {
+		t.Error("Flags byte array is not nil", to.Flags, from.Flags)
+	}
+
+	_, err = json.Marshal(from)
+	if err != nil {
+		t.Error("error when marshalling from struct")
+	}
+
+	_, err = json.Marshal(to)
+	if err != nil {
+		t.Error("error when marshalling from struct")
+	}
+}
+
 type TimeWrapper struct{
 	time.Time
 }