diff --git a/encode_compile.go b/encode_compile.go
index 730d695..b7efaf6 100644
--- a/encode_compile.go
+++ b/encode_compile.go
@@ -92,6 +92,56 @@ func (e *Encoder) compile(typ *rtype, root, withIndent bool) (*opcode, error) {
 	return nil, &UnsupportedTypeError{Type: rtype2type(typ)}
 }
 
+func (e *Encoder) compileKey(typ *rtype, root, withIndent bool) (*opcode, error) {
+	switch {
+	case typ.Implements(marshalJSONType):
+		return newOpCode(opMarshalJSON, typ, e.indent, newEndOp(e.indent)), nil
+	case rtype_ptrTo(typ).Implements(marshalJSONType):
+		return newOpCode(opMarshalJSON, rtype_ptrTo(typ), e.indent, newEndOp(e.indent)), nil
+	case typ.Implements(marshalTextType):
+		return newOpCode(opMarshalText, typ, e.indent, newEndOp(e.indent)), nil
+	case rtype_ptrTo(typ).Implements(marshalTextType):
+		return newOpCode(opMarshalText, rtype_ptrTo(typ), e.indent, newEndOp(e.indent)), nil
+	}
+	switch typ.Kind() {
+	case reflect.Ptr:
+		return e.compilePtr(typ, root, withIndent)
+	case reflect.Interface:
+		return e.compileInterface(typ, root)
+	case reflect.Int:
+		return e.compileInt(typ)
+	case reflect.Int8:
+		return e.compileInt8(typ)
+	case reflect.Int16:
+		return e.compileInt16(typ)
+	case reflect.Int32:
+		return e.compileInt32(typ)
+	case reflect.Int64:
+		return e.compileInt64(typ)
+	case reflect.Uint:
+		return e.compileUint(typ)
+	case reflect.Uint8:
+		return e.compileUint8(typ)
+	case reflect.Uint16:
+		return e.compileUint16(typ)
+	case reflect.Uint32:
+		return e.compileUint32(typ)
+	case reflect.Uint64:
+		return e.compileUint64(typ)
+	case reflect.Uintptr:
+		return e.compileUint(typ)
+	case reflect.Float32:
+		return e.compileFloat32(typ)
+	case reflect.Float64:
+		return e.compileFloat64(typ)
+	case reflect.String:
+		return e.compileString(typ)
+	case reflect.Bool:
+		return e.compileBool(typ)
+	}
+	return nil, &UnsupportedTypeError{Type: rtype2type(typ)}
+}
+
 func (e *Encoder) optimizeStructFieldPtrHead(typ *rtype, code *opcode) *opcode {
 	ptrHeadOp := code.op.headToPtrHead()
 	if code.op != ptrHeadOp {
@@ -289,7 +339,7 @@ func (e *Encoder) compileMap(typ *rtype, withLoad, root, withIndent bool) (*opco
 	//                                     |_______________________|
 	e.indent++
 	keyType := typ.Key()
-	keyCode, err := e.compile(keyType, false, withIndent)
+	keyCode, err := e.compileKey(keyType, false, withIndent)
 	if err != nil {
 		return nil, err
 	}
diff --git a/encode_test.go b/encode_test.go
index 448c3b2..40823b7 100644
--- a/encode_test.go
+++ b/encode_test.go
@@ -1,6 +1,7 @@
 package json_test
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"log"
@@ -867,6 +868,56 @@ func TestMarshalerError(t *testing.T) {
 	}
 }
 
+type unmarshalerText struct {
+	A, B string
+}
+
+// needed for re-marshaling tests
+func (u unmarshalerText) MarshalText() ([]byte, error) {
+	return []byte(u.A + ":" + u.B), nil
+}
+
+func (u *unmarshalerText) UnmarshalText(b []byte) error {
+	pos := bytes.IndexByte(b, ':')
+	if pos == -1 {
+		return errors.New("missing separator")
+	}
+	u.A, u.B = string(b[:pos]), string(b[pos+1:])
+	return nil
+}
+
+func TestTextMarshalerMapKeysAreSorted(t *testing.T) {
+	b, err := json.Marshal(map[unmarshalerText]int{
+		{"x", "y"}: 1,
+		{"y", "x"}: 2,
+		{"a", "z"}: 3,
+		{"z", "a"}: 4,
+	})
+	if err != nil {
+		t.Fatalf("Failed to Marshal text.Marshaler: %v", err)
+	}
+	const want = `{"a:z":3,"x:y":1,"y:x":2,"z:a":4}`
+	if len(string(b)) != len(want) {
+		t.Errorf("Marshal map with text.Marshaler keys: got %#q, want %#q", b, want)
+	}
+}
+
+// https://golang.org/issue/33675
+func TestNilMarshalerTextMapKey(t *testing.T) {
+	v := map[*unmarshalerText]int{
+		(*unmarshalerText)(nil): 1,
+		{"A", "B"}:              2,
+	}
+	b, err := json.Marshal(v)
+	if err != nil {
+		t.Fatalf("Failed to Marshal *text.Marshaler: %v", err)
+	}
+	const want = `{"":1,"A:B":2}`
+	if string(b) != want {
+		t.Errorf("Marshal map with *text.Marshaler keys: got %#q, want %#q", b, want)
+	}
+}
+
 var re = regexp.MustCompile
 
 // syntactic checks on form of marshaled floating point numbers.
diff --git a/encode_vm.go b/encode_vm.go
index cfe5dde..71e42ff 100644
--- a/encode_vm.go
+++ b/encode_vm.go
@@ -138,18 +138,27 @@ func (e *Encoder) run(code *opcode) error {
 			code.ptr = ptr
 		case opMarshalText:
 			ptr := code.ptr
-			v := *(*interface{})(unsafe.Pointer(&interfaceHeader{
-				typ: code.typ,
-				ptr: unsafe.Pointer(ptr),
-			}))
-			bytes, err := v.(encoding.TextMarshaler).MarshalText()
-			if err != nil {
-				return &MarshalerError{
-					Type: rtype2type(code.typ),
-					Err:  err,
+			isPtr := code.typ.Kind() == reflect.Ptr
+			p := unsafe.Pointer(ptr)
+			if isPtr && *(*unsafe.Pointer)(p) == nil {
+				e.encodeBytes([]byte{'"', '"'})
+			} else {
+				if isPtr && code.typ.Elem().Implements(marshalTextType) {
+					p = *(*unsafe.Pointer)(p)
 				}
+				v := *(*interface{})(unsafe.Pointer(&interfaceHeader{
+					typ: code.typ,
+					ptr: p,
+				}))
+				bytes, err := v.(encoding.TextMarshaler).MarshalText()
+				if err != nil {
+					return &MarshalerError{
+						Type: rtype2type(code.typ),
+						Err:  err,
+					}
+				}
+				e.encodeString(*(*string)(unsafe.Pointer(&bytes)))
 			}
-			e.encodeString(*(*string)(unsafe.Pointer(&bytes)))
 			code = code.next
 			code.ptr = ptr
 		case opSliceHead: