diff --git a/src/math/big/intmarsh.go b/src/math/big/intmarsh.go
index c1422e27107..ce429ffc11e 100644
--- a/src/math/big/intmarsh.go
+++ b/src/math/big/intmarsh.go
@@ -67,7 +67,10 @@ func (z *Int) UnmarshalText(text []byte) error {
 
 // MarshalJSON implements the json.Marshaler interface.
 func (x *Int) MarshalJSON() ([]byte, error) {
-	return x.MarshalText()
+	if x == nil {
+		return []byte("null"), nil
+	}
+	return x.abs.itoa(x.neg, 10), nil
 }
 
 // UnmarshalJSON implements the json.Unmarshaler interface.
diff --git a/src/math/big/intmarsh_test.go b/src/math/big/intmarsh_test.go
index f82956ceaf2..936669b380b 100644
--- a/src/math/big/intmarsh_test.go
+++ b/src/math/big/intmarsh_test.go
@@ -97,6 +97,20 @@ func TestIntJSONEncoding(t *testing.T) {
 	}
 }
 
+
+func TestIntJSONEncodingNil(t *testing.T) {
+	var x *Int
+	b, err := x.MarshalJSON()
+	if err != nil {
+		t.Fatalf("marshaling of nil failed: %s", err)
+	}
+	got := string(b)
+	want := "null"
+	if got != want {
+		t.Fatalf("marshaling of nil failed: got %s want %s", got, want)
+	}
+}
+
 func TestIntXMLEncoding(t *testing.T) {
 	for _, test := range encodingTests {
 		for _, sign := range []string{"", "+", "-"} {