diff --git a/entity/cliente.go b/entity/cliente.go
index c2e3d68..c0e421a 100644
--- a/entity/cliente.go
+++ b/entity/cliente.go
@@ -1,6 +1,9 @@
 package entity
 
-import "regexp"
+import (
+	"encoding/json"
+	"regexp"
+)
 
 type Cliente struct {
 	TipoPessoa                TipoPessoa `json:"tipoPessoa"`
@@ -14,6 +17,22 @@ type Cliente struct {
 	Endereco                  Endereco   `json:"endereco"`
 }
 
+func (c Cliente) MarshalJSON() ([]byte, error) {
+	tp := c.TipoPessoa
+	if string(c.TipoPessoa) == "" {
+		tp = tipoPessoa(c.CpfCnpj)
+	}
+
+	type Alias Cliente
+	return json.Marshal(&struct {
+		TipoPessoa TipoPessoa `json:"tipoPessoa"`
+		Alias
+	}{
+		Alias:      (Alias)(c),
+		TipoPessoa: tp,
+	})
+}
+
 func NewCliente(n string, d string, end Endereco) Cliente {
 	c := Cliente{
 		Nome:     n,
@@ -25,12 +44,14 @@ func NewCliente(n string, d string, end Endereco) Cliente {
 }
 
 func (c *Cliente) AutoTipoPessoa() {
-	switch len(onlyNumbers(c.CpfCnpj)) {
-	case 11:
-		c.TipoPessoa = PessoaFisica
-	case 14:
-		c.TipoPessoa = PessoaJuridica
+	c.TipoPessoa = tipoPessoa(c.CpfCnpj)
+}
+
+func tipoPessoa(doc string) TipoPessoa {
+	if len(onlyNumbers(doc)) == 14 {
+		return PessoaJuridica
 	}
+	return PessoaFisica
 }
 
 func onlyNumbers(s string) string {
diff --git a/entity/nfse.go b/entity/nfse.go
index 7d47f77..0204ac0 100644
--- a/entity/nfse.go
+++ b/entity/nfse.go
@@ -1,6 +1,7 @@
 package entity
 
 import (
+	"encoding/json"
 	"time"
 
 	"github.com/hashicorp/go-uuid"
@@ -26,6 +27,22 @@ type NFSe struct {
 	Servico           Servico    `json:"servico"`
 }
 
+func (n NFSe) MarshalJSON() ([]byte, error) {
+	var dataCompetencia string
+	if n.DataCompetencia != nil {
+		dataCompetencia = n.DataCompetencia.Format("2006-01-02T15:04Z")
+	}
+
+	type Alias NFSe
+	return json.Marshal(&struct {
+		Alias
+		DataCompetencia string `json:"dataCompetencia,omitempty"`
+	}{
+		Alias:           (Alias)(n),
+		DataCompetencia: dataCompetencia,
+	})
+}
+
 // NewNFSe cria um nova nota fiscal
 func NewNFSe(c Cliente, s Servico, v float64, a Ambiente) *NFSe {
 	return &NFSe{
diff --git a/entity/nfse_test.go b/entity/nfse_test.go
index 4ede7d4..b95c9bf 100644
--- a/entity/nfse_test.go
+++ b/entity/nfse_test.go
@@ -1,7 +1,9 @@
 package entity
 
 import (
+	"encoding/json"
 	"testing"
+	"time"
 )
 
 func TestNewNFSe(t *testing.T) {
@@ -18,3 +20,32 @@ func TestNewNFSe(t *testing.T) {
 		t.Errorf("EnviarPorEmail should not be false")
 	}
 }
+
+func TestNFSeMarshalJSON(t *testing.T) {
+	dataCompetencia, _ := time.Parse(time.RFC3339, "2006-01-02T15:04:05Z")
+
+	n := NFSe{
+		ID:              "e93ceafb-6fd9-4543-9106-e0aebac9ef8d",
+		Ambiente:        Homologacao,
+		DataCompetencia: &dataCompetencia,
+		Numero:          42,
+		EnviarPorEmail:  false,
+		ValorTotal:      3.47,
+		Cliente: Cliente{
+			Nome:     "John Doe",
+			Email:    "john@doe.com",
+			CpfCnpj:  "47142365471",
+			Endereco: Endereco{},
+		},
+	}
+	b, err := json.Marshal(n)
+	if err != nil {
+		t.Errorf("Expected no error, got %v", err)
+	}
+
+	expected := `{"id":"e93ceafb-6fd9-4543-9106-e0aebac9ef8d","ambienteEmissao":"Homologacao","numero":42,"enviarPorEmail":false,"valorTotal":3.47,"cliente":{"tipoPessoa":"F","nome":"John Doe","email":"john@doe.com","cpfCnpj":"47142365471","endereco":{"logradouro":"","numero":"","bairro":"","cep":"","cidade":"","uf":""}},"servico":{"cnae":"","codigoServicoMunicicio":"","descricao":"","aliquotaIss":0,"issRetidoFonte":false,"valorPis":0,"valorCofins":0,"valorCsll":0,"valorInss":0,"valorIr":0},"dataCompetencia":"2006-01-02T15:04Z"}`
+	if string(b) != expected {
+		t.Errorf("Expected '%s', got %s", expected, string(b))
+	}
+
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..81adc27
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,5 @@
+module github.com/padmoney/enotas-go
+
+go 1.15
+
+require github.com/hashicorp/go-uuid v1.0.2
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..9c04079
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,2 @@
+github.com/hashicorp/go-uuid v1.0.2 h1:cfejS+Tpcp13yd5nYHWDI6qVCny6wyX2Mt5SGur2IGE=
+github.com/hashicorp/go-uuid v1.0.2/go.mod h1:6SBZvOh/SIDV7/2o3Jml5SYk/TvGqwFJ/bN7x4byOro=