diff --git a/internal/nature.go b/internal/nature.go
index 2b757c4..049c066 100644
--- a/internal/nature.go
+++ b/internal/nature.go
@@ -13,3 +13,10 @@ const (
 	// Resgates ou alienação de unidades de participação ou liquidação de fundos de investimento
 	NatureG20 Nature = "G20"
 )
+
+func (n Nature) String() string {
+	if n == "" {
+		return "unknown"
+	}
+	return string(n)
+}
diff --git a/internal/nature_test.go b/internal/nature_test.go
new file mode 100644
index 0000000..dded2fb
--- /dev/null
+++ b/internal/nature_test.go
@@ -0,0 +1,38 @@
+package internal_test
+
+import (
+	"testing"
+
+	"github.com/nmoniz/any2anexoj/internal"
+)
+
+func TestNature_StringUnknow(t *testing.T) {
+	tests := []struct {
+		name   string
+		nature internal.Nature
+		want   string
+	}{
+		{
+			name: "return unknown",
+			want: "unknown",
+		},
+		{
+			name:   "return unknown",
+			nature: internal.NatureG01,
+			want:   "G01",
+		},
+		{
+			name:   "return unknown",
+			nature: internal.NatureG20,
+			want:   "G20",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := tt.nature.String()
+			if tt.want != got {
+				t.Fatalf("want %q but got %q", tt.want, got)
+			}
+		})
+	}
+}