diff --git a/README.md b/README.md
index ed44813..db8f31b 100644
--- a/README.md
+++ b/README.md
@@ -38,19 +38,22 @@ Replicator reads primary keys from MySQL table info and sync them automatically.
 Updating primary key in MySQL causes two Tarantool requests: delete an old row and insert a new one, because
 it is illegal to update primary key in Tarantool.
 
-### Force cast MySQL value
+### Custom mapping rules for columns
 
 Replicator can cast the value from MySQL to the required type 
 if your Tarantool schema does not comply with the MySQL schema.
 For example, MySQL column stores `bigint(20)` values, but Tarantool
 expects `unsigned`. 
-Without explicit casting you get an error, e.g.:
+Without explicit casting you will get an error, e.g.:
 > Tuple field 1 type does not match one required by operation: expected unsigned
 
 Supported types to cast to:
 * `unsigned`: try to cast any number to unsigned value.
 
-To enable this feature specify which column should be casted:
+If MySQL column stores `null` values, you can replace them by another value.
+It is useful when the space format is defined or you have an index on this field in Tarantool. 
+
+Custom column mapping configuration example:
 
 ```yaml
 ...
@@ -62,8 +65,14 @@ To enable this feature specify which column should be casted:
           - client_id
       dest:
         space: 'users'
-        cast:
-          client_id: 'unsigned'
+        column:
+          id:
+            cast: 'unsigned'
+          email:
+            on_null: 'my_default_value'
+          client_id: 
+            cast: 'unsigned'
+            on_null: 0
 ```
 
 ## Docker image
diff --git a/config/dev.conf.yml b/config/dev.conf.yml
index a1aa3d0..a6e92f7 100644
--- a/config/dev.conf.yml
+++ b/config/dev.conf.yml
@@ -45,3 +45,6 @@ replication:
           - email
       dest:
         space: 'users'
+        column:
+          email:
+            on_null: 'stub@mail.ru'
diff --git a/config/replicator.conf.yml b/config/replicator.conf.yml
index 86d22b5..58da7c3 100644
--- a/config/replicator.conf.yml
+++ b/config/replicator.conf.yml
@@ -45,3 +45,6 @@ replication:
           - email
       dest:
         space: 'users'
+        column:
+          email:
+            on_null: 'stub@mail.ru'
diff --git a/docker/mysql/init.d/init.sql b/docker/mysql/init.d/init.sql
index 9230a7c..e9b1aea 100644
--- a/docker/mysql/init.d/init.sql
+++ b/docker/mysql/init.d/init.sql
@@ -6,7 +6,7 @@ CREATE TABLE users
     username varchar(16)            not null,
     password varchar(254)           not null,
     name     varchar(50) default '' not null,
-    email    varchar(254)           not null
+    email    varchar(254)
 ) charset = utf8;
 
 CREATE TABLE logins
diff --git a/internal/bridge/attribute.go b/internal/bridge/attribute.go
index 4d7429f..da4d0d5 100644
--- a/internal/bridge/attribute.go
+++ b/internal/bridge/attribute.go
@@ -45,12 +45,13 @@ func castTypeFromString(str string) castType {
 
 // attribute represents MySQL column mapped to Tarantool.
 type attribute struct {
-	colIndex uint64   // column sequence number in MySQL table
-	tupIndex uint64   // attribute sequence number in Tarantool tuple
-	name     string   // unique attribute name
-	vType    attrType // value type stored in the column
-	cType    castType // value must be casted to this type
-	unsigned bool     // whether attribute contains unsigned number or not
+	colIndex uint64      // column sequence number in MySQL table
+	tupIndex uint64      // attribute sequence number in Tarantool tuple
+	name     string      // unique attribute name
+	vType    attrType    // value type stored in the column
+	cType    castType    // value must be casted to this type
+	onNull   interface{} // replace null by this value
+	unsigned bool        // whether attribute contains unsigned number or not
 }
 
 func newAttr(table *schema.Table, tupIndex uint64, name string) (*attribute, error) {
@@ -99,6 +100,10 @@ func (a *attribute) fetchValue(row []interface{}) (interface{}, error) {
 
 	value := row[a.colIndex]
 
+	if value == nil && a.onNull != nil {
+		value = a.onNull
+	}
+
 	if a.shouldCastToUInt64(value) {
 		v, err := toUint64(value)
 		if err != nil {
@@ -112,14 +117,14 @@ func (a *attribute) fetchValue(row []interface{}) (interface{}, error) {
 }
 
 func (a *attribute) shouldCastToUInt64(value interface{}) bool {
-	if value == nil {
-		return false
-	}
-
 	if a.cType == castUnsigned {
 		return true
 	}
 
+	if value == nil {
+		return false
+	}
+
 	if !a.unsigned {
 		return false
 	}
diff --git a/internal/bridge/attribute_test.go b/internal/bridge/attribute_test.go
index 490db4b..ce7d971 100644
--- a/internal/bridge/attribute_test.go
+++ b/internal/bridge/attribute_test.go
@@ -13,6 +13,7 @@ func Test_attribute_fetchValue(t *testing.T) {
 		name     string
 		vType    attrType
 		cType    castType
+		onNull   interface{}
 		unsigned bool
 	}
 	type args struct {
@@ -121,6 +122,40 @@ func Test_attribute_fetchValue(t *testing.T) {
 			want:    uint64(10),
 			wantErr: false,
 		},
+		{
+			name: "String_ReplaceOnNull",
+			fields: fields{
+				colIndex: 0,
+				tupIndex: 0,
+				name:     "email",
+				vType:    typeString,
+				cType:    castNone,
+				onNull:   "replacement",
+				unsigned: false,
+			},
+			args: args{
+				row: []interface{}{nil},
+			},
+			want:    "replacement",
+			wantErr: false,
+		},
+		{
+			name: "UnsignedNumber_ReplaceOnNull",
+			fields: fields{
+				colIndex: 0,
+				tupIndex: 0,
+				name:     "id",
+				vType:    typeNumber,
+				cType:    castUnsigned,
+				onNull:   102,
+				unsigned: false,
+			},
+			args: args{
+				row: []interface{}{nil},
+			},
+			want:    uint64(102),
+			wantErr: false,
+		},
 		{
 			name: "ColumnIndexEqualRowLen",
 			fields: fields{
@@ -164,6 +199,7 @@ func Test_attribute_fetchValue(t *testing.T) {
 				name:     tt.fields.name,
 				vType:    tt.fields.vType,
 				cType:    tt.fields.cType,
+				onNull:   tt.fields.onNull,
 				unsigned: tt.fields.unsigned,
 			}
 			got, err := a.fetchValue(tt.args.row)
diff --git a/internal/bridge/replicator.go b/internal/bridge/replicator.go
index 84a092c..3366db1 100644
--- a/internal/bridge/replicator.go
+++ b/internal/bridge/replicator.go
@@ -65,8 +65,6 @@ func New(cfg *config.Config, logger zerolog.Logger) (*Bridge, error) {
 		return nil, err
 	}
 
-	b.syncRulesAndCanalDump()
-
 	// We must use binlog full row image.
 	if err := b.canal.CheckBinlogRowImage("FULL"); err != nil {
 		return nil, err
@@ -97,7 +95,7 @@ func (b *Bridge) newRules(cfg *config.Config) error {
 	rules := make(map[string]*rule, len(cfg.Replication.Mappings))
 	for _, mapping := range cfg.Replication.Mappings {
 		source := mapping.Source
-		cast := mapping.Dest.Cast
+		colmap := mapping.Dest.Column
 
 		tableInfo, err := b.canal.GetTable(source.Schema, source.Table)
 		if err != nil {
@@ -109,8 +107,10 @@ func (b *Bridge) newRules(cfg *config.Config) error {
 			return fmt.Errorf("no primary keys found, schema: %s, table: %s", source.Schema, source.Table)
 		}
 		for _, pk := range pks {
-			typ := castTypeFromString(cast[pk.name])
-			pk.castTo(typ)
+			if m, ok := colmap[pk.name]; ok {
+				pk.castTo(castTypeFromString(m.Cast))
+				pk.onNull = m.OnNull
+			}
 		}
 
 		attrs := make([]*attribute, 0, len(source.Columns))
@@ -130,8 +130,11 @@ func (b *Bridge) newRules(cfg *config.Config) error {
 				if err != nil {
 					return err
 				}
-				typ := castTypeFromString(cast[name])
-				attr.castTo(typ)
+
+				if m, ok := colmap[name]; ok {
+					attr.castTo(castTypeFromString(m.Cast))
+					attr.onNull = m.OnNull
+				}
 
 				attrs = append(attrs, attr)
 			}
@@ -151,6 +154,7 @@ func (b *Bridge) newRules(cfg *config.Config) error {
 	}
 
 	b.rules = rules
+	b.syncRulesAndCanalDump()
 
 	return nil
 }
diff --git a/internal/bridge/replicator_test.go b/internal/bridge/replicator_test.go
index ad36e59..6fa824f 100644
--- a/internal/bridge/replicator_test.go
+++ b/internal/bridge/replicator_test.go
@@ -300,8 +300,11 @@ func (s *bridgeSuite) TestForceCast() {
 
 		cfg := *s.cfg
 		if wantErr {
+			cfg.Replication.Mappings = make([]config.Mapping, len(s.cfg.Replication.Mappings))
+			copy(cfg.Replication.Mappings, s.cfg.Replication.Mappings)
+
 			for i := range cfg.Replication.Mappings {
-				cfg.Replication.Mappings[i].Dest.Cast = nil
+				cfg.Replication.Mappings[i].Dest.Column = nil
 			}
 		}
 
@@ -343,6 +346,46 @@ func (s *bridgeSuite) TestForceCast() {
 	}
 }
 
+func (s *bridgeSuite) TestReplaceOnNull() {
+	t := s.T()
+	s.init(s.cfg)
+
+	go func() {
+		errors := s.bridge.Run()
+		for err := range errors {
+			assert.NoError(t, err)
+		}
+	}()
+
+	_, err := s.executeSQL("INSERT INTO city.users (username, password, name, email) VALUES (?, ?, ?, ?)", "alice", "12345", "Alice", nil)
+	require.NoError(t, err)
+
+	err = s.bridge.canal.CatchMasterPos(500 * time.Millisecond)
+	require.NoError(t, err)
+
+	require.Eventually(t, func() bool {
+		return s.hasSyncedData("users", 1)
+	}, 500*time.Millisecond, 50*time.Millisecond)
+
+	err = s.bridge.Close()
+	assert.NoError(t, err)
+
+	got, err := s.executeTNT(&tarantool.Select{
+		Space:    "users",
+		Iterator: tarantool.IterAll,
+	})
+	require.NoError(t, err)
+	require.NotNil(t, got)
+	require.NotEmpty(t, got.Data)
+	require.Len(t, got.Data, 1)
+	want := []interface{}{1, "alice", "12345", "null"}
+	gotTuple := got.Data[0]
+	require.Len(t, gotTuple, len(want))
+	for i, v := range want {
+		require.EqualValues(t, v, gotTuple[i])
+	}
+}
+
 func (s *bridgeSuite) TestReconnect() {
 	t := s.T()
 
diff --git a/internal/bridge/testdata/cfg.yml b/internal/bridge/testdata/cfg.yml
index 167467d..cdc1f0a 100644
--- a/internal/bridge/testdata/cfg.yml
+++ b/internal/bridge/testdata/cfg.yml
@@ -45,6 +45,9 @@ replication:
           - email
       dest:
         space: 'users'
+        column:
+          email:
+            on_null: 'null'
     - source:
         schema: 'city'
         table: 'logins'
@@ -54,5 +57,7 @@ replication:
           - latitude
       dest:
         space: 'logins'
-        cast:
-          attempts: 'unsigned'
\ No newline at end of file
+        column:
+          attempts:
+            cast: 'unsigned'
+            on_null: 0
\ No newline at end of file
diff --git a/internal/config/cfg.go b/internal/config/cfg.go
index e6ac154..1374363 100644
--- a/internal/config/cfg.go
+++ b/internal/config/cfg.go
@@ -134,11 +134,16 @@ type Mapping struct {
 	} `yaml:"source"`
 
 	Dest struct {
-		Space string            `yaml:"space"`
-		Cast  map[string]string `yaml:"cast"`
+		Space  string                   `yaml:"space"`
+		Column map[string]MappingColumn `yaml:"column"`
 	} `yaml:"dest"`
 }
 
+type MappingColumn struct {
+	Cast   string      `yaml:"cast"`
+	OnNull interface{} `yaml:"on_null,omitempty"`
+}
+
 func ReadFromFile(path string) (*Config, error) {
 	file, err := os.Open(path)
 	if err != nil {
diff --git a/internal/config/cfg_test.go b/internal/config/cfg_test.go
index f5e4e70..19aa97b 100644
--- a/internal/config/cfg_test.go
+++ b/internal/config/cfg_test.go
@@ -67,6 +67,20 @@ func TestReadFromFile_ValidPath(t *testing.T) {
 	assert.Equal(t, "users", mapping.Source.Table)
 	assert.Equal(t, []string{"username", "password", "email"}, mapping.Source.Columns)
 	assert.Equal(t, "users", mapping.Dest.Space)
-	assert.Len(t, mapping.Dest.Cast, 1)
-	assert.Contains(t, mapping.Dest.Cast, "attempts")
+	assert.Len(t, mapping.Dest.Column, 3)
+	columnMapping, ok := mapping.Dest.Column["attempts"]
+	if assert.True(t, ok) {
+		assert.Equal(t, "unsigned", columnMapping.Cast)
+		assert.Equal(t, 0, columnMapping.OnNull)
+	}
+	columnMapping, ok = mapping.Dest.Column["email"]
+	if assert.True(t, ok) {
+		assert.Equal(t, "", columnMapping.Cast)
+		assert.Equal(t, "", columnMapping.OnNull)
+	}
+	columnMapping, ok = mapping.Dest.Column["client_id"]
+	if assert.True(t, ok) {
+		assert.Equal(t, "unsigned", columnMapping.Cast)
+		assert.Nil(t, columnMapping.OnNull)
+	}
 }
diff --git a/internal/config/testdata/replicator.conf.yml b/internal/config/testdata/replicator.conf.yml
index bdbba28..f71ccbc 100644
--- a/internal/config/testdata/replicator.conf.yml
+++ b/internal/config/testdata/replicator.conf.yml
@@ -45,5 +45,11 @@ replication:
           - email
       dest:
         space: 'users'
-        cast:
-          attempts: 'unsigned'
+        column:
+          attempts:
+            cast: 'unsigned'
+            on_null: 0
+          email:
+            on_null: ''
+          client_id:
+            cast: 'unsigned'