diff --git a/encode_test.go b/encode_test.go
index 92f4304..59bb08f 100644
--- a/encode_test.go
+++ b/encode_test.go
@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"log"
 	"math"
+	"math/big"
 	"reflect"
 	"regexp"
 	"strconv"
@@ -2388,3 +2389,20 @@ func TestIssue324(t *testing.T) {
 		t.Fatalf("failed to encode. expected %q but got %q", expected, got)
 	}
 }
+
+func TestIssue339(t *testing.T) {
+	type T1 struct {
+		*big.Int
+	}
+	type T2 struct {
+		T1 T1 `json:"T1"`
+	}
+	v := T2{T1{Int: big.NewInt(10000)}}
+	b, err := json.Marshal(&v)
+	assertErr(t, err)
+	got := string(b)
+	expected := `{"T1":10000}`
+	if got != expected {
+		t.Errorf("unexpected result: %v != %v", got, expected)
+	}
+}
diff --git a/internal/encoder/compiler.go b/internal/encoder/compiler.go
index 1be01aa..de7323c 100644
--- a/internal/encoder/compiler.go
+++ b/internal/encoder/compiler.go
@@ -853,6 +853,9 @@ func (c *Compiler) implementsMarshalText(typ *runtime.Type) bool {
 }
 
 func (c *Compiler) isNilableType(typ *runtime.Type) bool {
+	if !runtime.IfaceIndir(typ) {
+		return true
+	}
 	switch typ.Kind() {
 	case reflect.Ptr:
 		return true