diff --git a/mp4.go b/mp4.go
index bc0286b..6523397 100644
--- a/mp4.go
+++ b/mp4.go
@@ -149,8 +149,8 @@ func (m metadataMP4) readAtomData(r io.ReadSeeker, name string, size uint32, pro
 		// "data" + size (4 bytes each)
 		b = b[8:]
 
-		if len(b) < 3 {
-			return fmt.Errorf("invalid encoding: expected at least %d bytes, for class, got %d", 3, len(b))
+		if len(b) < 4 {
+			return fmt.Errorf("invalid encoding: expected at least %d bytes, for class, got %d", 4, len(b))
 		}
 		class := getInt(b[1:4])
 		var ok bool