diff --git a/ros1msg/json_transcoder.go b/ros1msg/json_transcoder.go
index b735122..9d91114 100644
--- a/ros1msg/json_transcoder.go
+++ b/ros1msg/json_transcoder.go
@@ -258,11 +258,11 @@ func (t *JSONTranscoder) int64(w io.Writer, r io.Reader) error {
 }
 
 func (t *JSONTranscoder) uint64(w io.Writer, r io.Reader) error {
-	if _, err := io.ReadFull(r, t.buf[:4]); err != nil {
+	if _, err := io.ReadFull(r, t.buf[:8]); err != nil {
 		return fmt.Errorf("failed to read uint64: %w", err)
 	}
 	x := binary.LittleEndian.Uint64(t.buf[:8])
-	t.formattedNumber = strconv.AppendInt(t.formattedNumber, int64(x), 10)
+	t.formattedNumber = strconv.AppendUint(t.formattedNumber, x, 10)
 	if _, err := w.Write(t.formattedNumber); err != nil {
 		return fmt.Errorf("failed to write formatted uint64: %w", err)
 	}
diff --git a/ros1msg/json_transcoder_test.go b/ros1msg/json_transcoder_test.go
index 6cd8c0d..e268ba7 100644
--- a/ros1msg/json_transcoder_test.go
+++ b/ros1msg/json_transcoder_test.go
@@ -178,6 +178,7 @@ func TestJSONTranscoding(t *testing.T) {
 			err = transcoder.Transcode(buf, bytes.NewReader(c.input))
 			assert.Nil(t, err)
 			assert.Equal(t, c.expectedJSON, buf.String())
+			transcoder.buf = make([]byte, 8)
 		})
 	}
 }
@@ -309,8 +310,8 @@ func TestSingleRecordConversion(t *testing.T) {
 					converter: transcoder.uint64,
 				},
 			},
-			[]byte{0x07, 0x07, 0x07, 0x07, 0x07, 0x07, 0x07, 0x07},
-			`{"foo":506381209866536711}`,
+			[]byte{0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff},
+			`{"foo":18446744073709551615}`,
 		},
 		{
 			"float32",