diff --git a/main.go b/main.go
index 434cee0..2228cb9 100644
--- a/main.go
+++ b/main.go
@@ -13,9 +13,10 @@ const fname = "/home/rob/share/home-office/example.mp3"
 type (
 	AudioVersionId int
 	Layer          int
-	header         []byte
 )
 
+type Decoder struct{}
+
 const (
 	AudioVersionMPEG25 AudioVersionId = iota
 	AudioVersionReserved
@@ -54,6 +55,8 @@ var (
 	}
 )
 
+type header []byte
+
 func (h header) HasSyncWord() bool {
 	return h[0] == 0xff && h[1]&0xe0 == 0xe0
 }
@@ -99,6 +102,9 @@ func (h header) PaddingBytes() int {
 }
 
 func (h header) IsValid() bool {
+	if len(h) < 4 {
+		return false
+	}
 	if !h.HasSyncWord() {
 		return false
 	}
@@ -149,7 +155,6 @@ func (h header) BitRate() int {
 			j = 3
 		default:
 			return 0
-
 		}
 	}
 	return bitRates[j][h[2]>>4]
diff --git a/main_test.go b/main_test.go
index 8495d4a..52fff91 100644
--- a/main_test.go
+++ b/main_test.go
@@ -79,13 +79,49 @@ func TestLayer(t *testing.T) {
 }
 
 func TestPaddingBytes(t *testing.T) {
-	bytes := []byte{0xff, 0xe0, 0x2, 0}
-	h := header(bytes)
-	assert.Equal(t, 1, h.PaddingBytes())
+	testCases := []struct {
+		name       string
+		bytes      []byte
+		expPadding int
+	}{
+		{
+			"MPEG1 Layer3 128kbps 44.1k, no padding",
+			[]byte{0xff, 0xfa, 0x90, 0},
+			0,
+		},
+		{
+			"MPEG1 Layer3 128kbps 44.1k, padding",
+			[]byte{0xff, 0xfa, 0x92, 0},
+			1,
+		},
+		{
+			"MPEG2 Layer1 144kbps 24k, no padding",
+			[]byte{0xff, 0xf6, 0x94, 0},
+			0,
+		},
+		{
+			"MPEG2 Layer1 144kbps 24k, padding",
+			[]byte{0xff, 0xf6, 0x96, 0},
+			4,
+		},
+		{
+			"MPEG25 Layer2 56kbps 8k, no padding",
+			[]byte{0xff, 0xe4, 0x78, 0},
+			0,
+		},
+		{
+			"MPEG25 Layer2 56kbps 8k, padding",
+			[]byte{0xff, 0xe4, 0x7a, 0},
+			1,
+		},
+	}
 
-	bytes = []byte{0xff, 0xe0, 0x0, 0}
-	h = header(bytes)
-	assert.Equal(t, 0, h.PaddingBytes())
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			h := header(tc.bytes)
+			assert.Equal(t, tc.expPadding, h.PaddingBytes())
+		})
+	}
 }
 
 func TestIsProtected(t *testing.T) {
@@ -385,3 +421,49 @@ func TestSampleRate(t *testing.T) {
 		})
 	}
 }
+
+func TestLen(t *testing.T) {
+	testCases := []struct {
+		name   string
+		bytes  []byte
+		expLen int
+	}{
+		{
+			"MPEG1 Layer3 128kbps 44.1k, no padding",
+			[]byte{0xff, 0xfa, 0x90, 0},
+			417,
+		},
+		{
+			"MPEG1 Layer3 128kbps 44.1k, padding",
+			[]byte{0xff, 0xfa, 0x92, 0},
+			418,
+		},
+		{
+			"MPEG2 Layer1 144kbps 24k, no padding",
+			[]byte{0xff, 0xf6, 0x94, 0},
+			288,
+		},
+		{
+			"MPEG2 Layer1 144kbps 24k, padding",
+			[]byte{0xff, 0xf6, 0x96, 0},
+			292,
+		},
+		{
+			"MPEG25 Layer2 56kbps 8k, no padding",
+			[]byte{0xff, 0xe4, 0x78, 0},
+			1008,
+		},
+		{
+			"MPEG25 Layer2 56kbps 8k, padding",
+			[]byte{0xff, 0xe4, 0x7a, 0},
+			1009,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			h := header(tc.bytes)
+			assert.Equal(t, tc.expLen, h.Len())
+		})
+	}
+}