diff --git a/decode_interface.go b/decode_interface.go
index bc8d39a..0d0bfc2 100644
--- a/decode_interface.go
+++ b/decode_interface.go
@@ -29,7 +29,7 @@ func (d *interfaceDecoder) numDecoder(s *stream) decoder {
 
 var (
 	interfaceMapType = type2rtype(
-		reflect.TypeOf((*map[interface{}]interface{})(nil)).Elem(),
+		reflect.TypeOf((*map[string]interface{})(nil)).Elem(),
 	)
 )
 
@@ -38,12 +38,12 @@ func (d *interfaceDecoder) decodeStream(s *stream, p uintptr) error {
 	for {
 		switch s.char() {
 		case '{':
-			var v map[interface{}]interface{}
+			var v map[string]interface{}
 			ptr := unsafe.Pointer(&v)
 			d.dummy = ptr
 			if err := newMapDecoder(
 				interfaceMapType,
-				newInterfaceDecoder(d.typ),
+				newStringDecoder(),
 				newInterfaceDecoder(d.typ),
 			).decodeStream(s, uintptr(ptr)); err != nil {
 				return err
@@ -120,12 +120,12 @@ func (d *interfaceDecoder) decode(buf []byte, cursor int64, p uintptr) (int64, e
 	cursor = skipWhiteSpace(buf, cursor)
 	switch buf[cursor] {
 	case '{':
-		var v map[interface{}]interface{}
+		var v map[string]interface{}
 		ptr := unsafe.Pointer(&v)
 		d.dummy = ptr
 		dec := newMapDecoder(
 			interfaceMapType,
-			newInterfaceDecoder(d.typ),
+			newStringDecoder(),
 			newInterfaceDecoder(d.typ),
 		)
 		cursor, err := dec.decode(buf, cursor, uintptr(ptr))
diff --git a/decode_test.go b/decode_test.go
index c86565e..7c57b6b 100644
--- a/decode_test.go
+++ b/decode_test.go
@@ -204,7 +204,7 @@ func Test_Decoder(t *testing.T) {
 			var v interface{}
 			assertErr(t, json.Unmarshal([]byte(`{"a": 1, "b": "c"}`), &v))
 			assertEq(t, "interface.kind", "map", reflect.TypeOf(v).Kind().String())
-			m := v.(map[interface{}]interface{})
+			m := v.(map[string]interface{})
 			assertEq(t, "interface", `1`, fmt.Sprint(m["a"]))
 			assertEq(t, "interface", `c`, fmt.Sprint(m["b"]))
 		})
diff --git a/tagkey_test.go b/tagkey_test.go
index 8caf14f..29f5502 100644
--- a/tagkey_test.go
+++ b/tagkey_test.go
@@ -108,8 +108,8 @@ func TestStructTagObjectKey(t *testing.T) {
 		if err != nil {
 			t.Fatalf("Unmarshal(%#q) failed: %v", b, err)
 		}
-		for i, v := range f.(map[interface{}]interface{}) {
-			switch i.(string) {
+		for i, v := range f.(map[string]interface{}) {
+			switch i {
 			case tt.key:
 				if s, ok := v.(string); !ok || s != tt.value {
 					t.Fatalf("Unexpected value: %#q, want %v", s, tt.value)